Adjust the GH actions test workflows to run the complete test suite once with the cache turned on and once with the cache turned off.

The test run with caching turned on will run the tests twice and only run the tests which don't need isolation.
This should safeguard that the cache does not negatively influence sniff results.

The code coverage job will (should) run with caching turned _off_ to prevent tests not reaching all code paths due to cached results from previous tests short-circuiting things.

Note: the effect of caching on the test suite will be minimal as most tests will only execute a function once for a particular input/code snippet. Still good to safeguard/double-check though.