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

[bondhugula]

Summary

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

[aartbik]

I am okay if we come up with a single style, and I don't have a strong preference for - over _, but I would not say that _ right now is the exception.
I see many tests that have _ here and in integration (and yes, I have added many of those myself).

So what do others think?

[ftynse]

$ find mlir/test/ -name \*.mlir | grep _ | wc -l
101
$ find mlir/test/ -name \*.mlir | grep - | wc -l
551

- it is.