2.x: test to disallow anonymous inner classes

[akarnokd]

This PR adds an unit test that scans the compiled .class files for anonymous inner classes naming (i.e., dollar sign followed by a number). Since all the main, test and perf classes end up in the same place, the test has exceptions for file names containing Perf or Test.

The PR also fixes a few Checkstyle warnings from #5177 and fixes a couple classes from tests that don't have the word Test in their name and thus were detected.

[codecov]

Codecov Report

Merging #5183 into 2.x will increase coverage by 0.01%.
The diff coverage is 96.87%.

@@             Coverage Diff              @@
##                2.x    #5183      +/-   ##
============================================
+ Coverage     95.99%   96.01%   +0.01%     
- Complexity     5663     5664       +1     
============================================
  Files           621      621              
  Lines         40336    40340       +4     
  Branches       5616     5616              
============================================
+ Hits          38720    38731      +11     
+ Misses          637      629       -8     
- Partials        979      980       +1

Impacted Files	Coverage Δ	Complexity Δ
...va/io/reactivex/internal/util/ExceptionHelper.java	100% <ø> (ø)	15 <0> (ø)	❌
...rnal/operators/completable/CompletableTimeout.java	98% <100%> (+4%)	2 <0> (ø)	❌
...nal/operators/flowable/FlowableInternalHelper.java	100% <100%> (ø)	15 <0> (ø)	❌
...internal/operators/observable/ObservableDelay.java	100% <100%> (ø)	3 <0> (ø)	❌
...activex/internal/operators/single/SingleDelay.java	100% <100%> (ø)	2 <0> (ø)	❌
...ternal/operators/flowable/FlowableWindowTimed.java	83.67% <100%> (-0.23%)	4 <0> (ø)
...operators/observable/ObservableInternalHelper.java	89.07% <100%> (ø)	18 <0> (ø)	❌
...ators/flowable/FlowableDelaySubscriptionOther.java	100% <100%> (ø)	2 <0> (ø)	❌
...ctivex/internal/operators/single/SingleEquals.java	93.93% <95.83%> (+0.83%)	2 <0> (ø)	❌
.../operators/completable/CompletableConcatArray.java	93.75% <0%> (-6.25%)	2% <0%> (ø)
... and 42 more

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