2.x: Simplify JUnit tests with more appropriate assert methods

[slievrly]

Simplify unit tests by using the more appropriate JUnit assert methods.

[codecov]

Codecov Report

Merging #6546 into 2.x will decrease coverage by 0.04%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #6546      +/-   ##
============================================
- Coverage     98.27%   98.23%   -0.05%     
+ Complexity     6336     6334       -2     
============================================
  Files           677      677              
  Lines         45486    45486              
  Branches       6319     6319              
============================================
- Hits          44702    44682      -20     
- Misses          241      253      +12     
- Partials        543      551       +8

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Notification.java	93.93% <0%> (-6.07%)	22% <0%> (-1%)
...nternal/operators/observable/ObservableCreate.java	93.16% <0%> (-4.28%)	2% <0%> (ø)
...nternal/operators/parallel/ParallelReduceFull.java	91.08% <0%> (-3.97%)	2% <0%> (ø)
...ternal/operators/observable/ObservablePublish.java	94.73% <0%> (-3.51%)	12% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	93.23% <0%> (-2.42%)	2% <0%> (ø)
...ava/io/reactivex/processors/BehaviorProcessor.java	96.86% <0%> (-2.25%)	60% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	88.8% <0%> (-2.24%)	2% <0%> (ø)
.../io/reactivex/disposables/CompositeDisposable.java	98.14% <0%> (-1.86%)	39% <0%> (-1%)
...perators/single/SingleFlatMapIterableFlowable.java	95.83% <0%> (-1.67%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.93% <0%> (-1.64%)	2% <0%> (ø)
... and 22 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 0df3285...b7ba8ba. Read the comment docs.

[akarnokd]

How did you detect these?

Could you also post these PRs against 3.x too?

[slievrly]

@akarnokd I detected it through IDEA's Inspect Code plugin, and it works fine if it passes the unit test.
I can also submit this unit test simplified pr to 3.x