Speed up tests

[d70-t]

We need to find a way to speed up tests, as the catalog now (at least since #74) grows so large that re-testing everything might not be a sensible choice anymore.

Probably useful ideas:

• split tests based on data provider in order to run them in parallel and isolate errors if a provider is not reachable
• cache results of previous runs and retest only entries which have been changed since a last test run

[RobertPincus]

@d70-t I like both these ideas, but I'm not sure how to implement either of them.

[RobertPincus]

@d70-t @leifdenby I'm not a pytest guru but could this be accomplished with a Github matrix that listed each of the subcatalogs? Something like:

    strategy:
      matrix:
        platform:  [Atalante, ATR, barbados, Boreal, ...]

Tests will be run independent for each element of the platform vector. The invocation of pytest would then have to accept a keyword argument that restricted the tests to that platform... but I don't know how to do that.

[RobertPincus]

Closed by #76 though there's surely room for improvement.

[leifdenby]

@RobertPincus sorry for the slow reply here, and thanks for working on this!

A better approach I think would be to use pytest-xdist,

pytest -n <n_cpus>

There are two drawbacks with the merged approach 1) it assumes a folder structure of the repo and so if we add new directories these won't be included in the test, which is bad, and tests will break if folders are renamed and 2) it spawns a new testing instance for each platform which is quite wasteful (for every one of these there's a new conda environment created, pip installs, etc...). I think we should switch to pytest-xdist instead. I'll make a pull-request and see what number of parallel requests against AERIS we can get away with and we can merge it if you are both ok with it.