2.x: Fix Single.takeUntil, Maybe.takeUntil dispose behavior

[akarnokd]

Fix the dispose behavior of the Single.takeUntil and Maybe.takeUntil operators.

Tests were also added to the other 3 takeUntil variants (for Completable, it is delegated to amb).

Fixes: #6018

[codecov]

Codecov Report

Merging #6019 into 2.x will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #6019      +/-   ##
============================================
+ Coverage     98.27%   98.28%   +<.01%     
+ Complexity     6163     6161       -2     
============================================
  Files           659      659              
  Lines         44519    44521       +2     
  Branches       6201     6201              
============================================
+ Hits          43753    43757       +4     
+ Misses          231      229       -2     
  Partials        535      535

Impacted Files	Coverage Δ	Complexity Δ
...ernal/operators/maybe/MaybeTakeUntilPublisher.java	100% <100%> (ø)	2 <0> (ø)	⬇️
...vex/internal/operators/single/SingleTakeUntil.java	100% <100%> (ø)	2 <0> (ø)	⬇️
...nternal/operators/observable/ObservableCreate.java	95.72% <0%> (-3.42%)	2% <0%> (ø)
...tivex/internal/schedulers/TrampolineScheduler.java	96.1% <0%> (-2.6%)	6% <0%> (ø)
...nternal/operators/observable/ObservableWindow.java	98% <0%> (-2%)	3% <0%> (ø)
.../io/reactivex/disposables/CompositeDisposable.java	98.14% <0%> (-1.86%)	39% <0%> (-1%)
...java/io/reactivex/processors/PublishProcessor.java	98.19% <0%> (-1.81%)	42% <0%> (-1%)
...perators/single/SingleFlatMapIterableFlowable.java	96.66% <0%> (-1.67%)	2% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	94.02% <0%> (-1.5%)	2% <0%> (ø)
...internal/operators/flowable/FlowableSwitchMap.java	95.28% <0%> (-1.42%)	3% <0%> (ø)
... and 17 more

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