Auto merge of #50400 - ehuss:compiletest-revisions, r=alexcrichton
compiletest: Run revisions as independent tests.
Fixes #47604.
- The output of each test is now in its own directory.
- "auxiliary" output is now under the respective test directory.
- `stage_id` removed from filenames, and instead placed in the stamp file as a hash. This helps keep path lengths down for Windows.
In brief, the new layout looks like this:
```
<build_base>/<relative_dir>/<testname>.<revision>.<mode>/
stamp
<testname>.err
<testname>.out
a (binary)
auxiliary/lib<auxname>.dylib
auxiliary/<auxname>/<auxname>.err
auxiliary/<auxname>/<auxname>.out
```
(revision and mode are optional)