Rollup merge of #143825 - RalfJung:clippy-test-filter, r=llogiq

clippy: fix test filtering when TESTNAME is empty

Fixes rust-lang/rust#143824. Turns out bootstrap was just fine, the TESTNAME logic in clippy was wrong... I still made this a rustc PR as that's where I did all the debugging.

The bootstrap change is not really related, but it's comment-only so not worth a separate PR... adding the `test_args` is also part of what `prepare_cargo_test` would usually do so let's group the code properly.

Author: matthiaskrgr