Move some of the test information to TIP lines (#4007)

This has is a nice-to-have for me. Frequently I want to run a specific
test, and end up digging through output to be able to copy-paste the run
line. This uses TIP lines to inject the command into the file when using
AUTOUPDATE.

Note, one of the reasons I want this is because "bazel test
//toolchain/testing:file_test --test_output=all" has been regularly
exceeding bazel's output limit for me (workaround is either opening the
output file or specifying an obscure output limit flag), making it a
little harder for me to get the commands. However, frequently I'm adding
a file and want to iterate on it, so that's really the use case I have
in mind here.

toolchain/parse/testdata/function/definition/basic.carbon

@@ -3,6 +3,10 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 // AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/parse/testdata/function/definition/basic.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/parse/testdata/function/definition/basic.carbon
 
 fn F() {}
 extern fn G() {}

toolchain/parse/testdata/function/definition/builtin.carbon

@@ -3,6 +3,10 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 // AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/parse/testdata/function/definition/builtin.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/parse/testdata/function/definition/builtin.carbon
 
 fn F() = "builtin";
 

toolchain/parse/testdata/function/definition/fail_builtin.carbon

@@ -3,6 +3,10 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 // AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/parse/testdata/function/definition/fail_builtin.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/parse/testdata/function/definition/fail_builtin.carbon
 
 // CHECK:STDERR: fail_builtin.carbon:[[@LINE+4]]:18: ERROR: Expected builtin function name after `=`.
 // CHECK:STDERR: fn NotString() = banana;

toolchain/parse/testdata/function/definition/fail_identifier_in_statements.carbon

@@ -3,6 +3,10 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 // AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/parse/testdata/function/definition/fail_identifier_in_statements.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/parse/testdata/function/definition/fail_identifier_in_statements.carbon
 
 fn F() {
   // Note: this might become valid depending on the expression syntax. This test

toolchain/parse/testdata/function/definition/with_params.carbon

@@ -3,6 +3,10 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 // AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/parse/testdata/function/definition/with_params.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/parse/testdata/function/definition/with_params.carbon
 
 fn foo(bar: i64, baz: i64) {
   foo(baz, bar + baz);

toolchain/parse/testdata/function/definition/with_return_type.carbon

@@ -3,6 +3,10 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 // AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/parse/testdata/function/definition/with_return_type.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/parse/testdata/function/definition/with_return_type.carbon
 
 fn foo() -> f64 {
   return 42;