Investigate Jest's new custom equality matchers

[stephenh]

Historically Jest's toEqual has caused us grief b/c of recursively diffing entities down to the connection pool, which can end up pegging a CPU for ~several seconds even on a passing assertion.

We've written toMatchEntity to avoid this, but it requires teaching programmers when to use it.

Jest just landed custom equality matchers:

jestjs/jest#13654

So we can see if that helps. My suspicion is that toMatchEntity's diff output will still be easier to read, so it probably won't completely go away, but if we can de-suck toEqual, that would be great too.