3.x: Improve error messages in the test consumers

[akarnokd]

This PR improves the error messages reported in the TestObserver and TestSubscriber:

• assertValue(Predicate) now shows the value that didn't pass the predicate
• assertValueAt(index, T):
  • If the index is out of range, the message includes the valid range
  • the message now includes the index argument where the failure happened
• assertValueAt(index, Predicate):
  • If the index is out of range, the message includes the valid range
  • shows the value at index that didn't pass the predicate
  • the message now includes the index argument where the failure happened
• assertError(Predicate) is now worded to indicate the exception did not pass the predicate

In addition, the verification tests for the error messages were not actually testing the error message with assertThrows and have been updated.

Resolves #7125

[codecov]

Codecov Report

Merging #7126 (8ceb70f) into 3.x (67c1a36) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##                3.x    #7126      +/-   ##
============================================
- Coverage     99.57%   99.55%   -0.02%     
- Complexity     6671     6673       +2     
============================================
  Files           742      742              
  Lines         47267    47273       +6     
  Branches       6374     6376       +2     
============================================
- Hits          47066    47064       -2     
- Misses           89       94       +5     
- Partials        112      115       +3     

Impacted Files	Coverage Δ	Complexity Δ
.../reactivex/rxjava3/observers/BaseTestConsumer.java	100.00% <100.00%> (ø)	75.00 <3.00> (+5.00)
...tivex/rxjava3/internal/jdk8/ParallelCollector.java	94.49% <0.00%> (-3.67%)	2.00% <0.00%> (ø%)
...x/rxjava3/internal/operators/single/SingleAmb.java	96.61% <0.00%> (-3.39%)	9.00% <0.00%> (-1.00%)
...xjava3/internal/observers/FutureMultiObserver.java	96.61% <0.00%> (-3.39%)	27.00% <0.00%> (-1.00%)
...vex/rxjava3/internal/operators/maybe/MaybeAmb.java	96.96% <0.00%> (-3.04%)	10.00% <0.00%> (-1.00%)
...nternal/operators/parallel/ParallelReduceFull.java	91.08% <0.00%> (-1.99%)	2.00% <0.00%> (ø%)
...nternal/operators/observable/ObservableWindow.java	98.21% <0.00%> (-1.79%)	3.00% <0.00%> (ø%)
...nternal/operators/parallel/ParallelSortedJoin.java	98.49% <0.00%> (-1.51%)	2.00% <0.00%> (ø%)
.../operators/observable/ObservableCombineLatest.java	99.38% <0.00%> (-0.62%)	6.00% <0.00%> (ø%)
...a/io/reactivex/rxjava3/subjects/ReplaySubject.java	98.75% <0.00%> (-0.42%)	49.00% <0.00%> (ø%)
... and 6 more

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