2.x: BaseTestConsumer values() and errors() thread-safety clarifications

[akarnokd]

This PR adds clarifications to the BaseTestConsumer.values() and BaseTestConsumer.errors() method in regard to accessing elements while the upstream is still actively emitting events.

Related discussion: #5712

[codecov]

Codecov Report

Merging #5713 into 2.x will increase coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5713      +/-   ##
============================================
+ Coverage     96.28%   96.29%   +<.01%     
  Complexity     5822     5822              
============================================
  Files           634      634              
  Lines         41604    41604              
  Branches       5761     5761              
============================================
+ Hits          40060    40064       +4     
+ Misses          616      606      -10     
- Partials        928      934       +6

Impacted Files	Coverage Δ	Complexity Δ
.../java/io/reactivex/observers/BaseTestConsumer.java	99.65% <ø> (ø)	113 <0> (ø)	⬇️
...l/operators/observable/ObservableFlatMapMaybe.java	86.92% <0%> (-5.89%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.93% <0%> (-3.81%)	2% <0%> (ø)
.../io/reactivex/internal/schedulers/IoScheduler.java	92.47% <0%> (-2.16%)	9% <0%> (ø)
...rnal/operators/flowable/FlowableSkipLastTimed.java	95.91% <0%> (-2.05%)	2% <0%> (ø)
...a/io/reactivex/internal/queue/MpscLinkedQueue.java	98.03% <0%> (-1.97%)	17% <0%> (-1%)
.../io/reactivex/disposables/CompositeDisposable.java	97.24% <0%> (-1.84%)	39% <0%> (-1%)
...ternal/operators/flowable/FlowableSubscribeOn.java	96.61% <0%> (-1.7%)	2% <0%> (ø)
...rnal/subscriptions/DeferredScalarSubscription.java	98.46% <0%> (-1.54%)	28% <0%> (-1%)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	92.75% <0%> (-1.45%)	2% <0%> (ø)
... and 27 more

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