Add test coverage

[ReubenJ]

Closes #63

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[THinnerichs]

Which Julia versions do we want to test for? Shouldn't nightly already include 1.10?

[ReubenJ]

Nightly is the current state of the master branch, I believe. It's possible that something is broken in nightly that is working in 1.10 just because a new (broken) feature has been merged to master, or something like that.

I think we should be testing 1.8 and 1.10 at least since we say we support >1.8, and 1.10 is the latest stable release. If we want to be more thorough about it, we should test 1.9 and nightly, too, but either/both of those are probably overkill for now.

[THinnerichs]

Then we should rather do

version:
          - '1.8'
          - '1'
          - 'nightly'

which will automatically expand to the latest stable release.