Add test suite guidance to documentation

[cbegeman]

Add test suite descriptions to the documentation. This is intended to provide developers with a sense for how to design their tests for a particular suite and which suite to add test cases to.

Checklist

• [X] Developer's Guide has been updated
• [X] Documentation has been built locally and changes look as expected

[cbegeman]

@xylar, @alicebarthel, @lconlon, @mark-petersen and others: I'm hoping that this PR can serve as a discussion board for what our test suite philosophy is (especially for OMEGA). I put in some placeholder text for how many cores and what duration we expect for PR and nightly suites. We also talked about creating a release suite, which is not included here but could be.

@matthewhoffman and @trhille Right now, this is in a general section of the docs which implies that it applies to MALI testing too. MALI folks could weigh in if there is a similar testing philosophy, or I could make it clear that this only applies to ocean testing.

[cbegeman]

@mark-petersen @lconlon @trhille I've removed you as reviewers since we've gotten 3 reviews so far but you are still welcome to provide feedback if you like.

[cbegeman]

Thanks, @alicebarthel and @matthewhoffman, for giving this a look!

[xylar]

@cbegeman, I'll merge this though I had assigned it to you previously, which means you are the one with the power (and expectation) to merge it.

[cbegeman]

@xylar Thanks for merging. I'll keep an eye out for that next time.

[xylar]

@cbegeman, no problem. I just wanted you to know what the typical expectation is with who the PR is assigned to. That's not necessarily obvious.