Flaky unique inserter test about river

[brandur]

@bgentry Can you take a look at this one?

It got a little more complicated than I hoped. I believe the intermittency problem is what I described above in the pull request description, but I found when I went to push a fix that I was getting a massively failing test matrix.

Looking into it, that turned out to be because although our time stubs work to a certain extent, they're ignored completely when jobs are being inserted.

I was able to get this fixed up by adding created_at to inserts with a fall back to now(), but doing so makes the time stub system a little more elaborate. I tried a number of approaches, and this is the best one I could come up with, but it does change the time stub from a simple function to an interface that can be (1) stubbed, and (2) checked to see if it is stubbed.

It does add a little more code, but all the alternatives I can think of are worse. It's fairly important in specific cases that time be stubbable, and if job insertion always ignores a stub, then it's not really stubbable. I could possibly fix the test by inserting the job and the updating its created_at with a different query, but that seems pretty bad.

The good news is that we take three different time stub systems (one in limiter_test.go, the func approach in riverinternaltest, and the StubTime helper in riverinternaltest), and unify them into a single system.

[brandur]

Oops, posted that commented on the issue instead of the pull request. Going to delete and post again.