Have unit tests extends RxJavaTest - 2

[RomanWuattier]

This commit updates the unit tests of the following operators:

• flowable
• maybe
• observable
• disposable
• observer
• parallel
• processors
• schedulers
  and RxJava plugin.

Related: #6583

[akarnokd]

One of the ignored tests is failing the build

[codecov]

Codecov Report

Merging #6594 into 3.x will decrease coverage by <.01%.
The diff coverage is n/a.

@@ Coverage Diff @@ ## 3.x #6594 +/- ## ============================================ - Coverage 98.22% 98.21% -0.01%  + Complexity 6185 6184 -1  ============================================ Files 678 678 Lines 44908 44908 Branches 6214 6214 ============================================ - Hits 44111 44108 -3  + Misses 268 266 -2  - Partials 529 534 +5

Impacted Files	Coverage Δ	Complexity Δ
...nternal/operators/parallel/ParallelReduceFull.java	91.08% <0%> (-3.97%)	2% <0%> (ø)
...tivex/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
...ava/io/reactivex/processors/BehaviorProcessor.java	95.6% <0%> (-3.42%)	50% <0%> (-1%)
...rnal/operators/observable/ObservableSwitchMap.java	93.65% <0%> (-2.65%)	3% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...ternal/operators/observable/ObservablePublish.java	98.19% <0%> (-1.81%)	17% <0%> (-1%)
...nternal/operators/observable/ObservableCreate.java	97.43% <0%> (-1.71%)	2% <0%> (ø)
...tivex/internal/operators/single/SingleTimeout.java	98.33% <0%> (-1.67%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	95.65% <0%> (-1.09%)	2% <0%> (ø)
.../operators/mixed/FlowableSwitchMapCompletable.java	98.96% <0%> (-1.04%)	2% <0%> (ø)
... and 19 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 663e5a2...ee42dce. Read the comment docs.