2.x: increase timeout on parallel & completable tests

[akarnokd]

This PR increases the timeout on CompletableTest test methods and on ParallelFlowableTest.parallelismAndPrefetchAsync.

Related: #5154, #5179.

[codecov]

Codecov Report

Merging #5199 into 2.x will decrease coverage by 0.07%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##                2.x   #5199      +/-   ##
===========================================
- Coverage     95.98%   95.9%   -0.08%     
+ Complexity     5679    5673       -6     
===========================================
  Files           621     621              
  Lines         40611   40611              
  Branches       5632    5632              
===========================================
- Hits          38982   38950      -32     
- Misses          648     671      +23     
- Partials        981     990       +9

Impacted Files	Coverage Δ	Complexity Δ
...vex/internal/operators/single/SingleTakeUntil.java	86.88% <0%> (-8.2%)	2% <0%> (ø)
...ava/io/reactivex/processors/BehaviorProcessor.java	86.78% <0%> (-6.61%)	60% <0%> (-2%)
...a/io/reactivex/internal/util/QueueDrainHelper.java	58.86% <0%> (-5.68%)	31% <0%> (-4%)
.../operators/flowable/FlowableBlockingSubscribe.java	91.66% <0%> (-5.56%)	9% <0%> (-1%)
...rnal/subscriptions/DeferredScalarSubscription.java	93.84% <0%> (-4.62%)	27% <0%> (-1%)
...l/operators/observable/ObservableFlatMapMaybe.java	92.15% <0%> (-4.58%)	2% <0%> (ø)
...ernal/operators/maybe/MaybeTakeUntilPublisher.java	96% <0%> (-4%)	2% <0%> (ø)
...vex/internal/operators/flowable/FlowableTimer.java	92.85% <0%> (-3.58%)	2% <0%> (ø)
...vex/internal/operators/parallel/ParallelRunOn.java	94.41% <0%> (-3.05%)	6% <0%> (ø)
...nternal/operators/observable/ObservableCreate.java	96.42% <0%> (-2.68%)	2% <0%> (ø)
... and 36 more

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