Auto merge of #66524 - ecstatic-morse:compiletest-multiple-revisions, r=Centril

Support multiple revisions in `compiletest`

The `//[X]~` syntax filters errors for tests that are run across multiple cfgs with  `// revisions:`. This commit extends that syntax to accept `//[X,Y]~`, which will match multiple cfgs to the same error annotation. This is functionally the same as writing two comments, `//[X]~` and `//[Y]~`, but can fit on a single line.

While refactoring `compiletest` to support this, I also uncovered a small bug that was causing an incremental test to always pass, despite no errors being emitted.

r? @Centril