rustc: test: don't silently ignore bad benches

This is adequate because when a function has a type that isn't caught here,
that is, it has a single argument, but it *isn't* `&mut BenchHarness`, it
errors later on with:

     error: mismatched types: expected `fn(&mut test::BenchHarness)` but found
    `fn(int)` (expected &-ptr but found int)

which I consider acceptable.

Closes #12997