Fix pytest-xdist with session-scoped fixture

[tallamjr]

Currently, it is not possible to use more than --numprocessor=1 as with pytest-xdist the session scope is actually replicated and it does multiple loads of the data in fixture per worker.

There are ways to overcome this with code examples found here, but since we load a mix of numpy files and tf.data.Prefectdataset, it becomes a little complicated which for now doesn't really matter.

With this issue fix, tests will surely be able to run fast as we can then run in parallel, but for now must be run with single worker, otherwise there is sure to be memory problems

Refs:

• Session-Scoped Fixtures are not Session-Scoped with Pytest-Xdist pytest-dev/pytest-xdist#271
• Ability to execute certain tests sequentially while using -n (xdist) pytest-dev/pytest-xdist#385
• https://pypi.org/project/pytest-xdist/#id18
• https://medium.com/@kozyreva.hanna/pytest-xdist-with-session-scoped-fixture-b9884bd7bb29