Reorganize tests into dirs and rename numbered tests (#825)

There's a small update to update_checks.py to handle the recursive directories. Also, I'm only using one level of nesting in this PR but really no reason we can't do more. I'm just not sure what clustering is best right now.

As a pattern, I'm trying to name all failing tests `fail_*.carbon`.

executable_semantics/testdata/return/explicit_empty.carbon

@@ -0,0 +1,21 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// RUN: executable_semantics %s 2>&1 | \
+// RUN:   FileCheck --match-full-lines --allow-unused-prefixes=false %s
+// RUN: executable_semantics --trace %s 2>&1 | \
+// RUN:   FileCheck --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: executable_semantics %s
+// CHECK: result: 0
+
+package ExecutableSemanticsTest api;
+
+fn F() -> () {
+  return ();
+}
+
+fn main() -> i32 {
+  F();
+  return 0;
+}

executable_semantics/testdata/return/fail_auto.carbon

@@ -0,0 +1,20 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// RUN: not executable_semantics %s 2>&1 | \
+// RUN:   FileCheck --match-full-lines --allow-unused-prefixes=false %s
+// RUN: not executable_semantics --trace %s 2>&1 | \
+// RUN:   FileCheck --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: executable_semantics %s
+// CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/return/fail_auto.carbon:14: syntax error, unexpected AUTO
+
+package ExecutableSemanticsTest api;
+
+fn F() -> auto {
+  return 0;
+}
+
+fn main() -> i32 {
+  return F();
+}

executable_semantics/testdata/return/fail_explicit_with_no_return.carbon

@@ -0,0 +1,20 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// RUN: not executable_semantics %s 2>&1 | \
+// RUN:   FileCheck --match-full-lines --allow-unused-prefixes=false %s
+// RUN: not executable_semantics --trace %s 2>&1 | \
+// RUN:   FileCheck --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: executable_semantics %s
+// CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/return/fail_explicit_with_no_return.carbon:15: control-flow reaches end of function that provides a `->` return type without reaching a return statement
+
+package ExecutableSemanticsTest api;
+
+fn F() -> () {
+}
+
+fn main() -> i32 {
+  F();
+  return 0;
+}

executable_semantics/testdata/return/fail_explicit_with_plain_return.carbon

@@ -0,0 +1,21 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// RUN: not executable_semantics %s 2>&1 | \
+// RUN:   FileCheck --match-full-lines --allow-unused-prefixes=false %s
+// RUN: not executable_semantics --trace %s 2>&1 | \
+// RUN:   FileCheck --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: executable_semantics %s
+// CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/return/fail_explicit_with_plain_return.carbon:15: return; should provide a return value, to match the function's signature.
+
+package ExecutableSemanticsTest api;
+
+fn F() -> () {
+  return;
+}
+
+fn main() -> i32 {
+  F();
+  return 0;
+}

executable_semantics/testdata/return/fail_implicit_with_explicit_return.carbon

@@ -0,0 +1,21 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// RUN: not executable_semantics %s 2>&1 | \
+// RUN:   FileCheck --match-full-lines --allow-unused-prefixes=false %s
+// RUN: not executable_semantics --trace %s 2>&1 | \
+// RUN:   FileCheck --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: executable_semantics %s
+// CHECK: COMPILATION ERROR: {{.*}}/executable_semantics/testdata/return/fail_implicit_with_explicit_return.carbon:15: return (); should not provide a return value, to match the function's signature.
+
+package ExecutableSemanticsTest api;
+
+fn F() {
+  return ();
+}
+
+fn main() -> i32 {
+  F();
+  return 0;
+}

executable_semantics/testdata/return/implicit_with_no_return.carbon

@@ -0,0 +1,20 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// RUN: executable_semantics %s 2>&1 | \
+// RUN:   FileCheck --match-full-lines --allow-unused-prefixes=false %s
+// RUN: executable_semantics --trace %s 2>&1 | \
+// RUN:   FileCheck --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: executable_semantics %s
+// CHECK: result: 0
+
+package ExecutableSemanticsTest api;
+
+fn F() {
+}
+
+fn main() -> i32 {
+  F();
+  return 0;
+}

executable_semantics/testdata/return/implicit_with_plain_return.carbon

@@ -0,0 +1,21 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// RUN: executable_semantics %s 2>&1 | \
+// RUN:   FileCheck --match-full-lines --allow-unused-prefixes=false %s
+// RUN: executable_semantics --trace %s 2>&1 | \
+// RUN:   FileCheck --match-full-lines --allow-unused-prefixes %s
+// AUTOUPDATE: executable_semantics %s
+// CHECK: result: 0
+
+package ExecutableSemanticsTest api;
+
+fn F() {
+  return;
+}
+
+fn main() -> i32 {
+  F();
+  return 0;
+}