Test suite improvements

[VeckoTheGecko]

Currently the test suite takes 15+ minutes to run (windows is much more, closer to 30 mins). It would be good to reduce this, as well as bring other improvements to the test suite. I think code duplication, and "non-testing" (e.g., data setup) related code in the test suite is a pressing concern and makes it difficult to work with the test-suite.

We have 1112 tests, while xarray has 17354, yet xarray's test suite is 6 minutes. I think we can definitely improve here. From the top of my head (in order of what I think is most-> least pressing):

• Defining a set of standard fieldsets (and other parcels related data objects) to do testing on by making them into fixtures in a conftest.py file so its scoped across the test suite
  • We have the test_data folder with datasets. I think these are only used for integration tests, with a lot of unit tests generating fieldset/grids within the tests themselves
• Defining params appropriately to avoid unnecessary test duplication
• Scoping fixtures appropriately (reducing set up/tear down of fixtures that are used a lot). I think this is less of an issue.

I think it would be worth investigating any additional measures xarray does to help with performance.