Make testname selection into a callable function

[timholy]

This makes it much easier for CoverageBase to stay in sync with base. See #9493 (comment), which is still happening despite helpful efforts to keep it in sync (JuliaCI/CoverageBase.jl#2).

[tkelman]

Needs rebasing already. Will also cause a lot of churn in every open PR that adds a test. And needs to update CONTRIBUTING.md if the list of tests is going to move.

[tkelman]

whitespace here, possibly elsewhere too

[timholy]

Of open PRs labeled test, only one (#9674) appears to touch runtests.jl. I'd be fine if that one gets merged first, but it's not mergable as-is (even without this).

[andreasnoack]

#10180 and 4f4dd62 touch runtests.jl but the the rebases should hopefully be smooth.

[timholy]

Those are both in, so I'll rebase mine on top of those.

[tkelman]

@timholy I'm curious, did you (or someone else) manually cancel the pending AppVeyor builds of the older commit here, or is this something that AppVeyor is doing for us automatically now? I have code that checks and fails quickly when there are newer pending builds for the same PR, so manually cancelling only saves a minute or so in queue time.

[timholy]

I did it manually.

[tkelman]

Ah, okay. That isn't generally necessary unless you're really gung-ho about saving the minute or so that my code takes to stop the build.

The latest run here froze on AppVeyor during compilation of C code due to some recent makefile changes (see discussion here e85a1a5#commitcomment-9756164), so I restarted it.

[timholy]

Forgot about that; I won't worry about it in the future.

[timholy]

OK, this is green. If I don't hear any objections, I'll merge this very soon (to avoid further churn for other PRs that modify runtests.jl).