2.x: attempt to fix a few time-sensitive tests.

[akarnokd]

As of lately, the Travis CI runs much slower than usual and causes some time-sensitive tests to fail for some reason. This PR tries to figure out why this happens.

[codecov]

Codecov Report

Merging #5421 into 2.x will decrease coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5421      +/-   ##
============================================
- Coverage     96.12%   96.12%   -0.01%     
+ Complexity     5787     5785       -2     
============================================
  Files           630      630              
  Lines         41232    41232              
  Branches       5728     5728              
============================================
- Hits          39636    39635       -1     
- Misses          625      627       +2     
+ Partials        971      970       -1

Impacted Files	Coverage Δ	Complexity Δ
...vex/internal/operators/single/SingleTakeUntil.java	86.88% <0%> (-8.2%)	2% <0%> (ø)
...in/java/io/reactivex/subjects/BehaviorSubject.java	83.85% <0%> (-7.82%)	56% <0%> (ø)
.../main/java/io/reactivex/subjects/MaybeSubject.java	95.65% <0%> (-4.35%)	46% <0%> (-2%)
...reactivex/internal/operators/single/SingleAmb.java	96.36% <0%> (-3.64%)	9% <0%> (-1%)
.../internal/operators/flowable/FlowableInterval.java	94.44% <0%> (-2.78%)	3% <0%> (ø)
...java/io/reactivex/subjects/CompletableSubject.java	97.4% <0%> (-2.6%)	36% <0%> (-1%)
...ternal/operators/completable/CompletableUsing.java	97.61% <0%> (-2.39%)	4% <0%> (ø)
...main/java/io/reactivex/subjects/SingleSubject.java	95.23% <0%> (-2.39%)	38% <0%> (-1%)
...va/io/reactivex/internal/queue/SpscArrayQueue.java	97.61% <0%> (-2.39%)	22% <0%> (-1%)
...x/internal/operators/flowable/FlowablePublish.java	92.95% <0%> (-2.21%)	9% <0%> (ø)
... and 38 more

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