feat: starting generic testing framework

[henryiii]

Starting a generic testing helper framework. For a third party to test their implementation, they can subclass the provided UnitTest fixture and implement the required methods producing histograms.

Thoughts so far:

• I could make this a Mixin, which would workaround a possible issue with discovery if you import it directly into your test file instead of keeping a dotted name. But then typing is trickier, so I haven't done that (yet).
• I could provide the suggested data contents. I think this might ideally follow serialization.
• This uses unittest not just because that avoids a dependency, but also because pytest won't rewrite an assert that's not in a test file. It can be run with pytest and still prints nicely.

Closes #133.

[Copilot]

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