3.x: TestScheduler option to use onSchedule hook

[akarnokd]

The TestScheduler is a bit inconsistent regarding the use of RxJavaPlugins.onSchedule. It extends Scheduler so the scheduleDirect methods do invoke the hook whereas the worker's schedule methods don't.

This PR adds a constructor flag to enable the use of the hook in the worker's schedule methods as well. This addition should avoid issues with existing tests not expecting the hook to be invoked all of a sudden.

Resolves #7119

[codecov]

Codecov Report

Merging #7163 (0e74bd2) into 3.x (e657635) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##                3.x    #7163      +/-   ##
============================================
- Coverage     99.53%   99.53%   -0.01%     
- Complexity     6708     6711       +3     
============================================
  Files           745      745              
  Lines         47332    47345      +13     
  Branches       6376     6380       +4     
============================================
+ Hits          47113    47123      +10     
+ Misses          100       96       -4     
- Partials        119      126       +7     

Impacted Files	Coverage Δ	Complexity Δ
...va/io/reactivex/rxjava3/schedulers/Schedulers.java	100.00% <ø> (ø)	12.00 <0.00> (ø)
...io/reactivex/rxjava3/schedulers/TestScheduler.java	100.00% <100.00%> (ø)	13.00 <2.00> (+2.00)
...erators/completable/CompletableConcatIterable.java	95.91% <0.00%> (-4.09%)	2.00% <0.00%> (ø%)
...java3/internal/schedulers/TrampolineScheduler.java	97.40% <0.00%> (-2.60%)	6.00% <0.00%> (ø%)
.../operators/observable/ObservableFlatMapSingle.java	94.44% <0.00%> (-2.39%)	2.00% <0.00%> (ø%)
...rnal/operators/flowable/FlowableFlatMapSingle.java	94.18% <0.00%> (-2.33%)	2.00% <0.00%> (ø%)
...tivex/rxjava3/internal/jdk8/ParallelCollector.java	96.33% <0.00%> (-1.84%)	2.00% <0.00%> (ø%)
...a3/internal/operators/flowable/FlowableCreate.java	97.41% <0.00%> (-1.62%)	6.00% <0.00%> (ø%)
...ternal/operators/completable/CompletableMerge.java	97.29% <0.00%> (-1.36%)	2.00% <0.00%> (ø%)
...operators/flowable/FlowableConcatMapScheduler.java	99.20% <0.00%> (-0.40%)	4.00% <0.00%> (ø%)
... and 8 more

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