Tests: Move tests for external modules into its own directory.

[OmarEid451]

In order to make the files slightly more manageable I moved various test files for external science modules (matplotlib, scipy, etc) as well as django into their own folder.

[bwoodsend]

There's much more to it than just moving some files around I'm afraid.

• You'll need to update CI so that this new suite actually runs. Namely here and here.

• Likewise in the developer docs.

• And here so that just running pytest runs all three suites.

• There are also resource lookups which will need to be adjusted.

[rokm]

Is it really worth moving those files around?

[Legorooj]

While in principle I have no issues with moving tests so they're in more logical locations, I'd rather have it done in one fell swoop - one big PR to clean up the entire test suite.

[rokm]

I suppose split between stdlib and 3rd party libraries would make sense if we wanted to split the CI run in two steps.

Otherwise, I don't see much benefit in splitting the test suite between stdlib and 3rd party libraries; not least of all because sometimes it is difficult to draw the line between the two (i.e., if the tests are grouped by functionality rather than library).

One change I think would make sense is to move stuff from tests/functional/test_hooks into parent tests/functional, so that all tests are in the same directory, and sub-directories contain only data and auxiliary files needed by the tests.