Remove remaining uses of mlir-runner from the test suite

Cf. the comments on https://github.com/google/heir/pull/1466

mlir-runner adds a ton of build dependencies because the upstream bazel rules pull in every MLIR dialect and transform, which also pulls in a lot of extra LLVM code (e.g., for gpu targets and whatnot).

If we convert the remaining uses of mlir-runner to lower all the way to LLVM and run C-based tests, we will probably reduce the time to build tests by maybe 30% (10 minutes)?