Write tests for legacy-data migration

[gnprice]

At launch time, the implementation of #1070 that we'll have (mainly from #1588) won't include automated tests of its own.

That logic got extensive manual testing, plus careful scrutiny of the code. But as the codebase changes over time, the informativeness of that manual testing degrades — unless repeated, which is a lot of work. That also creates a barrier to editing the relevant code, for fear of either breaking it or repeating a bunch of manual testing.

So it'd be good to write some tests for it.