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
- Moves most parts to //testing/lit_test to be consistent with //testing/file_test.
- Separates the autoupdate script out because it's shared between lit_test and file_test now, not lit-specific.
- Renames scripts to autoupdate_testdata (or autoupdate_lit_testdata for explorer's extra) to be more consistent with the non-lit-specific setup.
- Switches from execv to subprocess.call to head off a subtle issue regarding execution of multiple scripts, which we're likely to want in the future. Mostly in this PR because everything was already being touched.
- Removes autoupdate's dependency on merge_output in order to (a) better support the division of lit and non-lit logic and (b) remove a subprocess, for reasons similar to file_test's removal of subprocesses.
Jon Ross-Perkins