[mlir][linalg][transform][python] Clean up _ext.py test. (#66469)

This PR cleans up the test of the mix-ins of this dialect. Most of the
character diff is due to factoring out the creation of the the top-level
sequence into a decorator. This decorator siginficantly shortens the
definition of the individual tests and can be used in all but one test,
where the top-level op is a PDL op. The only functional diff is due to
the fact that the decator uses `transform.any_op` instead of
`pdl.operation` for the type of the root handle. The only remaining
usages of the PDL dialects is now in the test a PDL-related op.

Author: ingomueller-net