2.x: Fix flaky sample() backpressure test, improve coverage

[akarnokd]

Use processors to trigger the MissingBackpressureException case instead of the timer, which could fire prematurely and stop the sequence before the first item was sent by the BehaviorProcessor.

Also small improvements to the code coverage of FlowableSample.

Fixes #6253.

[codecov]

Codecov Report

Merging #6254 into 2.x will increase coverage by 0.07%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##               2.x    #6254      +/-   ##
===========================================
+ Coverage     98.2%   98.28%   +0.07%     
- Complexity    6202     6206       +4     
===========================================
  Files          667      667              
  Lines        44889    44889              
  Branches      6216     6216              
===========================================
+ Hits         44082    44117      +35     
+ Misses         260      247      -13     
+ Partials       547      525      -22

Impacted Files	Coverage Δ	Complexity Δ
...ex/internal/subscriptions/SubscriptionArbiter.java	94.91% <0%> (-2.55%)	46% <0%> (-1%)
...ava/io/reactivex/processors/BehaviorProcessor.java	96.86% <0%> (-2.25%)	60% <0%> (ø)
...java/io/reactivex/processors/PublishProcessor.java	98.19% <0%> (-1.81%)	42% <0%> (-1%)
...tivex/internal/operators/single/SingleTimeout.java	98.33% <0%> (-1.67%)	2% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	88.8% <0%> (-1.5%)	2% <0%> (ø)
...nternal/operators/observable/ObservableCreate.java	97.43% <0%> (-0.86%)	2% <0%> (ø)
...l/operators/observable/ObservableTimeoutTimed.java	99.15% <0%> (-0.85%)	3% <0%> (ø)
...internal/operators/flowable/FlowableObserveOn.java	96.65% <0%> (-0.56%)	3% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.93% <0%> (-0.55%)	2% <0%> (ø)
...main/java/io/reactivex/subjects/ReplaySubject.java	97.69% <0%> (-0.42%)	50% <0%> (ø)
... and 25 more

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