Add a CircleCI step which tries to verify that the dummy API files have landed before running QUnit

[cgolubi1]

• Partially addresses Tweak our CircleCI setup to get it "just so" #2342
• I can't prove this works or that i've diagnosed the problem correctly --- my test CircleCI run passed on the first try, and we know we're getting spurious QUnit failures pretty often in CircleCI (i'd say at least 1/5 of the time?), so if we merge this and don't see more spurious failures, or if we merge this and ever see it need to sleep once before continuing, we should feel very confident in it. And if we merge this and do see another spurious failure, we can go back to the drawing board.

[blackshadowshade]

I'm happy to try this. I'll rebase my pull requests (when I have time) and then I'll rerun them all, including the one I haven't been able to get to pass yet. We'll see if this makes a difference.