Small re-organization of writing recording and sorting extractors tests

[h-mayorquin]

Now that we have move further to have more isolated testing with in the spikeinterface module I think it would be a good idea to separate the previous tests that were added mainly to support writing extractors from spikeextractors (see catalystneuro/nwb-conversion-tools#182) into its own testing module. This PR does that by creating a new testing module called test_spikeextractors_to_nwb which contains all the old tests and keeps the new tests on test_si.

There are two advantages of this:

1. It would divide the tests for the function that they are providing (testing writing spikeinterface objects vs testing writing spikeextractors objects)
2. The test in the test_si module are way more isolated and unittest-like so they run faster. It is therefore faster to discover a failure without the need to run the slow tests of the spikeextractors which are some of the slowest tests onr our suite (see [Feature]: tests take a long time to run locally nwb-conversion-tools#449)

[CodyCBakerPhD]

Sure, fine by me. Will make cleanup easier when we eventually retire it altogether