tests: re-enable caching tests and more cleanups in tests/dsl

[romanc]

Description

This PR looks at tests under tests/dsl and does some cleanup work. In particular

• Re-enable tests in tests_caches.py: They were skipped because MPI is always defined (since it's a non-optional import)
• Fix tests in tests_caches.py: Tests haven't been running since we moved away from python 3.8 (because the python version was hard-coded in there ...)
• Move make_storage into a utility module (used from two test modules)
• Run tests in tests/dsl/test_stencil_factory.py with more than one backend
• Adding type hints and "inlining" single parameter sets.

Just to disambiguate: Some tests (the ones inside tests/mpi) rely on MPI being changed to None in case of a serial run:

NDSL/tests/mpi/mpi_comm.py

Lines 1 to 6 in 2986b45

	from mpi4py import MPI
	if MPI.COMM_WORLD.Get_size() == 1:
	# not run as a parallel test, disable MPI tests
	MPI = None

Tests outside test/mpi import MPI from mpi4py directly and thus check the world size themselves. Might be worth cleaning this up anyway ... or - you know - use pytest.marker instead. Feel free to leave this open if you want to discuss this.

How Has This Been Tested?

Running tests locally.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas: N/A
• I have made corresponding changes to the documentation: N/A
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules: N/A
• New check tests, if applicable, are included: N/A