Convert SKIP_SLOW_TESTS to its own decorator

[JesseWeinstein]

No description provided.

[sampsyo]

Great! Feel free to merge at will.

I would advocate for slow_test for PEP-8 compliance (instead of matching unittest's camelCase aberration), but to each their own.

[diego-plan9]

Apologies for "reviving" this pull request, but I'm wondering if there are some guidelines or metric on when to use the slow_test decorator, @JesseWeinstein? I'm in the process of writing some more tests for the edit plugin at #1846 and would love some clarification on it's intended usage!

[JesseWeinstein]

Happy to clarify! I added it to tests that took a perceptible time to run -- i.e. all 106 non-slow tests in test_ui.py took 4.217 seconds for me to run just now (i.e. 0.04 seconds per test), while one of the slow ones, CompletionTest, took 1.121 seconds just by itself. So I'd say probably add @slow_test to tests that take more than a tenth of a second to run, and certainly add it to tests that take more than a whole second to run.

[diego-plan9]

Thanks for the hints, sounds like a quite sensible guideline!