Migrate to pytest

[bjlittle]

This pull-request migrates away from the home-brew nose runner implementation to pytest.

Note that, the tests may be now run from the command line using nox or pytest. However, when using pytest from the CLI the iris environment variable OVERRIDE_TEST_DATA_REPOSITORY requires to be also configured.

I've not used the pytest-xdist plugin, as I didn't deem it in scope ATM; the existing runtime of the tests is relatively short.

In the future, we should look to use filterwarnings to control and manage the expected warnings raised during a test run.

[pp-mo]

Totally good with this !

Just one question : are the lockfiles re-resolved or manually edited ?
I can see they lose nose + gain a few new things, but no version change to existing packages. I guess that must be "ok", but possibly not optimal ?

[bjlittle]

@pp-mo The lock files were resolved using conda-lock, and result in nose being dropped and pytest being added.

We've recently updated the lock files in #419 (to remove pep8) so I wouldn't expect there to be much change in the other packages.

So this all seem totally okay to me 👍

[stephenworsley]

Looks good!

[bjlittle]

@pp-mo and @stephenworsley

Awesome, thanks guys! 🥳