[mlir][linalg][transform][python] Verify modules in mix-in tests.

The tests of the mix-in classes of the Python bindings currently passed
even if the ops constructed by the mix-ins under test failed to verify.
This is because the assembled IR is still printed in generic form even
if it does not verify, and the `CHECK` statements are formulated in such
a lenient way that they also match that generic form. This patch adds
explicit verification to the decorator that is used for all test
functions.

Reviewed By: springerm

Differential Revision: https://reviews.llvm.org/D157790