Move some inotify-tests to run on all backends; test that state is cleaned up after...

Move some inotify-tests to run on all backends; test that state is cleaned up after Remove (#494)

This moves a number of tests from backend_inotify_test.go to
fsnotify_test.go, as they're not really inotify-specific. In particular,
it moves the "stress test", which creates a bunch of events/files. It
also expands this test to (potentially) create many more than just 1,000
files, depending on how many the system will allow.

Unfortunately these tests seem pretty flaky on kqueue platforms, where
they're allowed to fail for now (failing the test won't fail the test
run). This seems to expose some existing limits/problems that need to
fixed in a future PR.

Also test that the internal state is cleaned up with TestRemoveState().
The Windows backend doesn't have a test for it (or rather, it doesn't
run) as it *doesn't* clean the state properly, but I found it too
confusing to fix 🤷 Need to spend some time on that in the future.

Reorder/rename some GitHub Actions test runs to show nicer in the UI.

Fixes #42
Fixes #268