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

---

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 de4ce93...5262542. Read the comment docs.