TimestampProviders and TestScheduler "run mode"

[benlesh]

Currently, TestScheduler "run mode" will patch all schedulers by default. This means that everywhere we were only using schedulers as TimestampProviders, will use the test scheduler's currently time when now() is called.

However... we're going to be breaking people's tests with a move to use Date or performance as default TimestampProviders.

I see two solutions:

1. Have TestScheduler temporarily monkey patch Date.now and performance.now to accommodate bare uses of things like animationFrames, etc.
2. Create very, very light wrappers around Date.now and Performance.now that are used internally (but not exported, yet) that we monkey patch when we are in run mode.

I'm inclined to say that #2 is the best answer, and the least likely to break others.

[benlesh]

Core team picks option 2.

[cartant]

Pretty sure this is closed by #5607.