Setup testing

[kaitj]

• Adds testing tiers (nothing implemented here yet)
• Updated actions workflow triggering slow + e2e (separate flag) tests manually
• Added fixture markers and initial fixtures
• Added initial testing README (partially generated to provide context)

I think some of these things may change as we actually start to write tests but this sets up an initial structure.

Closes #13

[nx10]

I'm curious if this will be possible - maybe with the dry runner it will

[kaitj]

I think it will be, at least for unit testing...integration testing might take longer.

[nx10]

I meant the 90% min

[kaitj]

We shall find out. Arbitrary numbers 🤷‍♂️

[nx10]

Maybe keep abridged version in README and move full guide to seperate markdown doc

[nx10]

Or full guide as section in contributing md

[kaitj]

Alternatively, make each less relevant subsection a dropdown. Not the biggest fan of that, but 🤷. I'll take a look to see what I did for other repos - probably not a lot 😂

[nx10]

Do we need to make sure test data is not included in wheels/built package or is tests/ excluded anyway

[kaitj]

Do we need to make sure test data is not included in wheels/built package or is tests/ excluded anyway

It should be excluded in the build, but can double check.

EDIT: Only src is built with the way this repo is setup.

[kaitj]

Shortened the test README - a lot of unnecessary fluff there.
This also adds an initial implementation for the testing across OS' though this is currently happening for all tests. Should we break down each tier of testing into its own job and then combine the coverage later on (or we can fix this later on too).

[nx10]

LGTM 🚀