This is really part of #2811, but is extracted out to allow a little review in parallelism because #2811 expects #2813. Getting this in will allow migration of toolchain tests, whereas #2811 is focused on explorer tests. For explorer test timing information, see #2811.
The syntax being used for matching deliberately mirrors the `FileCheck` setup, partly for compatibility if something changes, partly so there's nothing new to learn, partly so that we don't need to build more test updating.
Individual tests look like:
```
[ RUN ] ParseAndExecuteTestFile.explorer/parse_and_execute/testdata/assert/convert.carbon
To test this file alone, run:
bazel test //explorer/parse_and_execute:file_test.subset --test_arg=explorer/parse_and_execute/testdata/assert/convert.carbon
[ OK ] ParseAndExecuteTestFile.explorer/parse_and_execute/testdata/assert/convert.carbon (202 ms)
```
The printed command line is intended to assist developers in debugging a single test, particularly when sharding the main test. The use of a single `.subset` target means the total number of targets is constant even as the number of test files increases, which may be important for some `bazel` execution environments. I plan to make similar changes to the `glob_sh_run` implementation so that we have consistent setups, i.e. that we no longer create target-per-file scaling risks.
This uses `native_test` to share the test binary, avoiding re-linking if files are individually run.
Investigation did reveal a mistake where STDOUT/STDERR wasn't prefixed on empty output lines; this PR fixes that mistake, so that output is fully covered.
Co-authored-by: Chandler Carruth <chandlerc@gmail.com>
Jon Ross-PerkinsandChandler Carruth