Clean up test for -f{,no-}implicit-modules-uses-lock

@arichardson pointed out in post-commit review for
https://reviews.llvm.org/D95583 (b714f73d) that `-verify` has an
optional argument that works a lot like `FileCheck`'s `-check-prefix`.
Use it to simplify the test for `-fno-implicit-modules-use-lock`!