2.x: Fix TrampolineScheduler not calling RxJavaPlugins.onSchedule(), add tests for all schedulers.

[artem-zinnatullin]

Noticed during figuring out how #5734 works.

[codecov]

Codecov Report

Merging #5747 into 2.x will increase coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #5747      +/-   ##
============================================
+ Coverage     96.24%   96.27%   +0.02%     
+ Complexity     5833     5832       -1     
============================================
  Files           634      634              
  Lines         41644    41644              
  Branches       5768     5768              
============================================
+ Hits          40082    40094      +12     
+ Misses          633      616      -17     
- Partials        929      934       +5

Impacted Files	Coverage Δ	Complexity Δ
...tivex/internal/schedulers/TrampolineScheduler.java	94.87% <100%> (ø)	6 <1> (ø)	⬇️
...l/operators/observable/ObservableFlatMapMaybe.java	84.96% <0%> (-7.85%)	2% <0%> (ø)
.../operators/flowable/FlowableBlockingSubscribe.java	91.89% <0%> (-5.41%)	9% <0%> (-1%)
...rnal/subscribers/SinglePostCompleteSubscriber.java	94.87% <0%> (-5.13%)	14% <0%> (-1%)
...tivex/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
.../internal/operators/flowable/FlowableInterval.java	94.44% <0%> (-2.78%)	3% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	93.71% <0%> (-2.42%)	2% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...vex/internal/operators/flowable/FlowableCache.java	92.61% <0%> (-1.35%)	7% <0%> (ø)
...ternal/operators/completable/CompletableMerge.java	96.42% <0%> (-1.2%)	2% <0%> (ø)
... and 21 more

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