mk/tests.mk: fix typo "srcrustllvm"
mk/tests.mk: remove mention of nonexistent files
auto merge of #8166 : emillon/rust/clean-tests-mk, r=brson

Hello,

While looking at `tests.mk` I noticed two errors:

  - there's a `srcrustllvm` instead of `src/rustllvm`
  - some filtered out files don't exist anymore

These two commits fix these issues. Thanks!