compiletest: `./x test <test-name>.stderr` (or existing non-`.rs` file) should warn or error

Noticed by @WaffleLapkin, if you write

./x test tests/ui/parser/bad-let-else-statement.stderr

then compiletest will silently pass but run 0 tests, because bad-let-else-statement.stderr is a tracked stderr file associated with the actual .rs ui test but not a test itself. This might be surprising or give a false sense of security to the person who's running the test suite thinking that the test passed, whereas it actually didn't even run.

• If the path doesn't exist, compiletest will report the non-existent test error.
• However, if the path does exist, but it's not an actual test (whatever consitutes a "test" file under the relevant test suite), compiletest currently seems to silently ignore and pass.

We should probably warn or error if the path exists (and has an extension) but is not a valid test file.