[mlir][python] Segment MLIR Python test dialect to avoid testonly dependency.

With https://reviews.llvm.org/rG14c9207063bb00823a5126131e50c93f6e288bd3, the build is broken with -DMLIR_INCLUDE_TESTS=OFF. This patch fixes the build and we may want to do a better fix to the layering in a followup.

Differential Revision: https://reviews.llvm.org/D112560