Refactor lit testing to generate per-file tests. (#1063)

This switches to generating per-file targets such as `//executable_semantics/testdata:tuple/equality_false.carbon.test`, instead of one test covering all files. Essentially this relies on bazel instead of lit to handle test parallelization.

executable_semantics/BUILD

@@ -2,7 +2,6 @@
 # Exceptions. See /LICENSE for license information.
 # SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
-load("//bazel/testing:lit_test.bzl", "lit_test")
 load("@mypy_integration//:mypy.bzl", "mypy_test")
 
 package(default_visibility = [
@@ -23,16 +22,6 @@ cc_binary(
     ],
 )
 
-lit_test(
-    name = "executable_semantics_lit_test",
-    data = [
-        ":executable_semantics",
-        "@llvm-project//llvm:FileCheck",
-        "@llvm-project//llvm:not",
-    ],
-    test_dir = "testdata",
-)
-
 py_binary(
     name = "gen_rtti",
     srcs = ["gen_rtti.py"],

executable_semantics/README.md

@@ -111,11 +111,7 @@ To explain this boilerplate:
 ### Useful commands
 
 -   `./update_checks.py` -- Updates expected output.
--   `bazel test :executable_semantics_lit_test --test_output=errors` -- Runs
-    tests and prints any errors.
--   `bazel test :executable_semantics_lit_test --test_output=errors --test_arg=--filter=basic_syntax/.*`
-    -- Only runs tests in the `basic_syntax` directory; `--filter` is a regular
-    expression.
+-   `bazel test ... --test_output=errors` -- Runs tests and prints any errors.
 
 ## Experimental feature: Delimited Continuations
 

executable_semantics/testdata/BUILD

@@ -0,0 +1,15 @@
+# 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
+
+load("//bazel/testing:lit.bzl", "glob_lit_tests")
+
+glob_lit_tests(
+    data = [
+        "//executable_semantics",
+        "@llvm-project//llvm:FileCheck",
+        "@llvm-project//llvm:not",
+    ],
+    driver = "lit.cfg.py",
+    test_file_exts = ["carbon"],
+)

executable_semantics/update_checks.py

@@ -29,7 +29,7 @@ def _get_tests() -> Set[str]:
     tests = set()
     for root, _, files in os.walk(_TESTDATA):
         for f in files:
-            if f == "lit.cfg.py":
+            if f in {"lit.cfg.py", "BUILD"}:
                 # Ignore the lit config.
                 continue
             if os.path.splitext(f)[1] == ".carbon":