[Feature] improve testing set-up

[alessandrofelder]

Is your feature request related to a problem? Please describe. By default, not all tests are run, neither in a local development environment nor on CI. You need to specify --runlocal --runslow to pytest.

Describe the solution you'd like Either all tests are run always, or all tests are run by default locally (but not on CI)

Additional context

• See https://github.com/brainglobe/brainrender/pull/284#discussion_r1409381890
• also consider that using pooch and shipping example data in mega-fix should render at least the --runlocal part obsolete.

[adamltyson]

FYI we do have a convention here: https://brainglobe.info/community/developers/testing.html#long-running-tests

[adamltyson]

Just bumping this. Basically all the "slow" tests fail, but they're rarely run, so they may as well not be there.