fix(Cache): Correct concurrent request coalescing

[bartelink]

Fixes the condition for the swap, which has a race condition that can mean delivering the value we've just literally determined to be too old
Reduce a test timeout to hopefully reduce flakiness
Also fixes an oversight: even if a concurrent request was too old for the maxAge, it can still serve as the base state for an incremental load (not a big deal in the normal case; only the very first load would be suboptimal)

[bartelink]

And yes, the tests should not be using Task.Delay to engineer overlaps and/or expirations. The more correct (and, in retrospect, obvious) approach is to maximise use of TimeProvider and/or events, in order to make them quicker, more deterministic and correct (would really appreciate a PR regarding that; it's unfortunately too far down my overall list of priorities at the moment...)