Create a base test class and have all unit tests extend it

[RomanWuattier]

This commit creates the RxJavaTest class defining a default
timeout to 5 minutes. An ignored test announce itself for each
running class extending it preventing Travis CI from killing the
build.

Have Completable tests extend from RxJavaTest.
Have Disposable tests extend from RxJavaTest.
Have Exception tests extend from RxJavaTest

Related: #6583

[codecov]

Codecov Report

Merging #6589 into 3.x will increase coverage by 0.05%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #6589      +/-   ##
============================================
+ Coverage     98.16%   98.21%   +0.05%     
- Complexity     6183     6187       +4     
============================================
  Files           678      678              
  Lines         44908    44908              
  Branches       6214     6214              
============================================
+ Hits          44083    44107      +24     
+ Misses          278      268      -10     
+ Partials        547      533      -14

Impacted Files	Coverage Δ	Complexity Δ
...ternal/operators/completable/CompletableMerge.java	96.42% <0%> (-2.39%)	2% <0%> (ø)
...tivex/internal/operators/single/SingleTimeout.java	98.33% <0%> (-1.67%)	2% <0%> (ø)
...rnal/operators/observable/ObservableSwitchMap.java	94.7% <0%> (-1.59%)	3% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	96.25% <0%> (-1.07%)	5% <0%> (ø)
...ex/internal/operators/flowable/FlowableCreate.java	96.12% <0%> (-0.97%)	6% <0%> (ø)
...nternal/operators/observable/ObservableCreate.java	98.29% <0%> (-0.86%)	2% <0%> (ø)
...perators/single/SingleFlatMapIterableFlowable.java	97.5% <0%> (-0.84%)	2% <0%> (ø)
...main/java/io/reactivex/subjects/ReplaySubject.java	96.68% <0%> (-0.42%)	50% <0%> (ø)
...internal/operators/flowable/FlowableConcatMap.java	98.5% <0%> (-0.38%)	6% <0%> (ø)
.../java/io/reactivex/processors/ReplayProcessor.java	98.97% <0%> (+0.4%)	52% <0%> (ø)	⬇️
... and 17 more

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[RomanWuattier]

Thanks for updating the description and the review. I amended the commit to meet the new PR description.
I will cut the issue into a few more pull requests for simple readability reasons.

[akarnokd]

@RomanWuattier do you want to do the rest of the test classes in a separate PR?

[RomanWuattier]

@akarnokd yes, you can merge this one when you want to.

[RomanWuattier]

Thanks!