[MachO] Fix test on llvm-x86_64-debian-dylib builder

Make the `--implicit-check-not` account for the file extension as well.
This will still fail if we ever have a builder with `.dylib` in its
name, and we probably want a more robust solution, but this addresses
the immediate issue.

Fixes https://github.com/llvm/llvm-project/issues/75850
Fixes https://github.com/llvm/llvm-project/issues/75910