3.x: Remove methods from test consumers, make internal versions

[akarnokd]

Remove methods from the standard test consumers based on feedback in 6153 comment.

Some test support has been moved into the io.reactivex.testsupport package in the form of TestObserverEx and TestSubscriberEx.

Resolves: #6153

[codecov]

Codecov Report

Merging #6526 into 3.x will decrease coverage by <.01%.
The diff coverage is 80%.

@@ Coverage Diff @@ ## 3.x #6526 +/- ## ============================================ - Coverage 98.16% 98.15% -0.01%  + Complexity 6251 6155 -96  ============================================ Files 675 675 Lines 45011 44783 -228 Branches 6211 6180 -31 ============================================ - Hits 44183 43956 -227  Misses 286 286 + Partials 542 541 -1

Impacted Files	Coverage Δ	Complexity Δ
...main/java/io/reactivex/observers/TestObserver.java	96.92% <ø> (-3.08%)	25 <0> (-25)
...x/internal/operators/flowable/FlowablePublish.java	98.52% <ø> (ø)	18 <0> (ø)	⬇️
.../java/io/reactivex/subscribers/TestSubscriber.java	97.61% <ø> (-1.66%)	35 <0> (-22)
...ternal/operators/observable/ObservablePublish.java	100% <100%> (ø)	18 <0> (ø)	⬇️
src/main/java/io/reactivex/Completable.java	100% <100%> (ø)	117 <0> (ø)	⬇️
src/main/java/io/reactivex/Single.java	100% <100%> (ø)	147 <0> (ø)	⬇️
...ors/completable/CompletableAndThenCompletable.java	96.66% <100%> (ø)	2 <0> (ø)	⬇️
src/main/java/io/reactivex/Maybe.java	100% <100%> (ø)	171 <0> (ø)	⬇️
.../java/io/reactivex/observers/BaseTestConsumer.java	97.23% <50%> (-2.77%)	69 <0> (-47)
...tivex/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
... and 28 more

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