2.x: improve coverage of some classes (no functional changes)

[akarnokd]

Improve the coverage of some classes, plus re-add the unit test that validates the StrictSubscriber.

Only test changes, no functional changes.

[codecov]

Codecov Report

Merging #5310 into 2.x will increase coverage by 0.07%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5310      +/-   ##
============================================
+ Coverage     96.06%   96.14%   +0.07%     
- Complexity     5765     5782      +17     
============================================
  Files           630      630              
  Lines         41146    41146              
  Branches       5714     5714              
============================================
+ Hits          39528    39558      +30     
+ Misses          655      623      -32     
- Partials        963      965       +2

Impacted Files	Coverage Δ	Complexity Δ
...internal/operators/completable/CompletableAmb.java	94.91% <0%> (-5.09%)	10% <0%> (-1%)
...vex/internal/operators/single/SingleTakeUntil.java	86.88% <0%> (-4.92%)	2% <0%> (ø)
...nternal/operators/parallel/ParallelSortedJoin.java	91.3% <0%> (-3.63%)	2% <0%> (ø)
.../internal/operators/flowable/FlowableInterval.java	93.33% <0%> (-3.34%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableTakeLastTimed.java	96.29% <0%> (-2.78%)	2% <0%> (ø)
...java/io/reactivex/subjects/CompletableSubject.java	94.8% <0%> (-2.6%)	35% <0%> (-1%)
.../operators/observable/ObservableSkipLastTimed.java	95.23% <0%> (-2.39%)	2% <0%> (ø)
...main/java/io/reactivex/subjects/SingleSubject.java	97.61% <0%> (-2.39%)	39% <0%> (-1%)
...operators/observable/ObservableConcatMapEager.java	97.26% <0%> (-2.19%)	2% <0%> (ø)
...perators/single/SingleFlatMapIterableFlowable.java	96.66% <0%> (-1.67%)	2% <0%> (ø)
... and 30 more

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