test: Adds `test-(slow|fast)` options

[dangotbanned]

While working on #3547, I've been running the test suite a bunch to identify bottlenecks.

This PR provides

• tools to identify, skip or run only slow tests
• some new, previously repeated mark decorators
• misc refactoring of tests using the above
• docs explaining intended usage

Impact?

• Locally, this covers most tests that take 2-10 secs.
• In total, 354/1671 tests are skipped
• reduced runtime 40 -> 20 secs

Important

The default behaviour is unchanged:

# Runs all tests
>>> hatch test 

# Runs only slow tests
>>> hatch run test-slow

# Runs all tests (that are not marked @slow)
>>> hatch run test-fast

[dangotbanned]

• https://github.com/vega/altair/pull/3555#commits-pushed-a4f6c20

These were some drive-by fixes of typing issues, fixing them here in case this stays in review for a while They would show on mypy as well, but are hidden due to https://microsoft.github.io/pyright/#/mypy-comparison?id=type-checking-unannotated-code

[dangotbanned]

Really neat! :) I learned a lot while reading this. Also great to have more convenient decorators for skipping tests re pyarrow and vlc.

Thanks @binste!

I've found when reading the pytest docs that they have a lot of info - but can be overwhelming at times. The docs I've added here are as much for my own benefit; as they are to simplify the test writing of others