dillon1000/react

`act()` - s / flushPassiveEffects / Scheduler.unstable_flushWithoutYielding (#15591)

* s/flushPassiveEffects/unstable_flushWithoutYielding

a first crack at flushing the scheduler manually from inside act(). uses unstable_flushWithoutYielding(). The tests that changed, mostly replaced toFlushAndYield(...) with toHaveYielded(). For some tests that tested the state of the tree before flushing effects (but still after updates), I replaced act() with bacthedUpdates().

* ugh lint

* pass build, flushPassiveEffects returns nothing now

* pass test-fire

* flush all work (not just effects), add a compatibility mode

of note, unstable_flushWithoutYielding now returns a boolean much like flushPassiveEffects

* umd build for scheduler/unstable_mock, pass the fixture with it

* add a comment to Shcduler.umd.js for why we're exporting unstable_flushWithoutYielding

* run testsutilsact tests in both sync/concurrent modes

* augh lint

* use a feature flag for the missing mock scheduler warning

I also tried writing a test for it, but couldn't get the scheduler to unmock. included the failing test.

* Update ReactTestUtilsAct-test.js

- pass the mock scheduler warning test,
- rewrite some tests to use Scheduler.yieldValue
- structure concurrent/legacy suites neatly

* pass failing tests in batchedmode-test

* fix pretty/lint/import errors

* pass test-build

* nit: pull .create(null) out of the act() call