Revisit pytest-xdist for parallel testing

[agriyakhetarpal]

As discussed in #193 (comment). @BradyPlanden mentions that the order of the tests to be scheduled is different with the use of pytest-xdist, which, to my understanding, causes issues with threading.

A solution to this problem could be to run the flaky plotly tests in serial mode, but it needs further investigation.

[BradyPlanden]

For reference, I've added a xdist implementation in #204. It work out well to implement at the same time as the redistribution of the unit tests.