[MLIR] NFC. Rename test cases in test/mlir-cpu-runner per convention

Test case files at most places in MLIR uses hyphens and not underscores.
A counter-pattern was somehow started to use underscores in some places.
Rename test cases in test/mlir-cpu-runner to use hyphens so that it's
consistent at least within its directory.

Differential Revision: https://reviews.llvm.org/D114672