3.x: Fix Junit 4.13 deprecated API use

[akarnokd]

JUnit 4.13 deprecated the use of ExpectedException in favor of assertThrows.

[codecov]

Codecov Report

Merging #6813 into 3.x will decrease coverage by 0.05%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #6813      +/-   ##
============================================
- Coverage     98.28%   98.23%   -0.06%     
+ Complexity     6390     6386       -4     
============================================
  Files           715      715              
  Lines         46498    46498              
  Branches       6371     6371              
============================================
- Hits          45702    45677      -25     
- Misses          283      295      +12     
- Partials        513      526      +13

Impacted Files	Coverage Δ	Complexity Δ
...l/operators/observable/ObservableFlatMapMaybe.java	88.73% <0%> (-4.23%)	2% <0%> (ø)
...java3/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.44% <0%> (-2.91%)	2% <0%> (ø)
...tivex/rxjava3/disposables/CompositeDisposable.java	98.14% <0%> (-1.86%)	39% <0%> (-1%)
...tivex/rxjava3/internal/jdk8/ParallelCollector.java	89.9% <0%> (-1.84%)	2% <0%> (ø)
...internal/operators/flowable/FlowableFromArray.java	96.61% <0%> (-1.7%)	3% <0%> (ø)
...ctivex/rxjava3/internal/util/QueueDrainHelper.java	98.61% <0%> (-1.39%)	57% <0%> (-1%)
...ava3/internal/operators/parallel/ParallelJoin.java	92.04% <0%> (-1.14%)	3% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	95.18% <0%> (-1.07%)	5% <0%> (ø)
...rxjava3/internal/schedulers/ExecutorScheduler.java	95.89% <0%> (-0.92%)	10% <0%> (ø)
... and 10 more

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