2.x: improve BaseTestConsumer with awaitCount & timeout

[akarnokd]

This PR enhances the timeout detection and reporting of the TestSubscriber/TestObserver base class BaseTestConsumer:

• awaitCount(int atLeast [, Runnable waitStrategy [, timeout]]): wait until at least the given amount of onNext events have been observed or the upstream terminated.
• Enhance awaitX methods to set a timeout flag.
• Show the timeout! string in the assertion failures set by the awaitX methods.
• Show the disposed! string in the assertion failures if the consumer has been cancelled/disposed.
• assertTimeout and assertNoTimeout to assert explicitly after an awaitX method.
• isTimeout and clearTimeout to check and clear the flag status.

In addition, the internal array that collects the items has been replaced with a new custom List implementation: VolatileSizeArrayList that makes sure size() is a volatile read and happens before the committing the size in another thread due to add(). (In theory, one shouldn't cast values() unconditionally to ArrayList anyway.)

[codecov]

Codecov Report

Merging #5155 into 2.x will decrease coverage by -0.07%.
The diff coverage is 99.06%.

@@             Coverage Diff              @@
##                2.x    #5155      +/-   ##
============================================
- Coverage     95.88%   95.82%   -0.07%     
- Complexity     5606     5645      +39     
============================================
  Files           620      621       +1     
  Lines         39856    39960     +104     
  Branches       5604     5610       +6     
============================================
+ Hits          38217    38291      +74     
- Misses          654      672      +18     
- Partials        985      997      +12

Impacted Files	Coverage Δ	Complexity Δ
...reactivex/internal/util/VolatileSizeArrayList.java	100% <100%> (ø)	30 <30> (?)
.../java/io/reactivex/observers/BaseTestConsumer.java	99.61% <98.18%> (+1.56%)	108 <16> (+15)	✅
...rnal/subscriptions/ArrayCompositeSubscription.java	86.66% <0%> (-13.34%)	15% <0%> (-3%)
...rnal/subscribers/SinglePostCompleteSubscriber.java	87.17% <0%> (-12.83%)	13% <0%> (-2%)
...a/io/reactivex/processors/SerializedProcessor.java	87.23% <0%> (-10.64%)	25% <0%> (-3%)
...al/operators/observable/ObservableSampleTimed.java	90% <0%> (-6.67%)	3% <0%> (ø)
...ternal/operators/observable/ObservablePublish.java	89.18% <0%> (-6.31%)	9% <0%> (-1%)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	89.55% <0%> (-5.98%)	2% <0%> (ø)
...rnal/subscriptions/DeferredScalarSubscription.java	93.84% <0%> (-4.62%)	27% <0%> (-1%)
.../internal/operators/maybe/MaybeTakeUntilMaybe.java	95.91% <0%> (-4.09%)	2% <0%> (ø)
... and 38 more

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