I believe this broke when I was changing path handling.
Bad:
```
bazel test //testing/file_test:file_test_base_test.subset --test_arg=/usr/local/...elided.../execroot/carbon/bazel-out/k8-fastbuild/bin/testing/file_test/file_test_base_test.runfiles/carbon/testing/file_test/example.carbon
```
Good:
```
bazel test //testing/file_test:file_test_base_test.subset --test_arg=testing/file_test/example.carbon
```
The intent of this change is that instead of paths looking like `explorer/testdata/foo/bar.carbon` (repo-relative), they're now just `bar.carbon` (local). The consequence is that paths should be a bit more durable in various environments, and just paths should be shorter and easier to read.
The explorer's prelude is an exception to this since it comes from data, rather than being the test target. Due to the change in approaches, it needs the regex again.
Uses #2829
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-Perkins
