compiletest should remove `.stderr` for nonexistent revisions

[varkor]

I added a normal ui test, generated a .stderr, then later added revisions to the test so that revA.stderr and revB.stderr files were generated. I forgot to manually remove the old .stderr file, and so I ended up committing it, despite it now being unused. It would be nice if compiletest automatically caught this situation (it probably also applies to when the list of revisions is changed).

[jyn514]

It might be easier to make this a tidy check.

[jieyouxu]

@rustbot claim