Diamond operators in tests

[dvolkovv]

Hello, in this pull request i've changed all IDE marked explicit types with diamond operator. Affected packages is internal/ , internal/operators/.
There is one test fail in CompletableTest.repeatNormal , but diamond is not the cause, there is last stack entry:
java.lang.AssertionError: expected:<6> but was:<5>
at org.junit.Assert.fail(Assert.java:88)

This PR is part of #6767 issue resolving.

[codecov]

Codecov Report

Merging #6788 into 3.x will decrease coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #6788      +/-   ##
============================================
- Coverage     98.24%   98.23%   -0.01%     
  Complexity     6286     6286              
============================================
  Files           696      696              
  Lines         45390    45390              
  Branches       6233     6233              
============================================
- Hits          44592    44589       -3     
  Misses          277      277              
- Partials        521      524       +3

Impacted Files	Coverage Δ	Complexity Δ
.../operators/observable/ObservableFlatMapSingle.java	88.09% <0%> (-4.77%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	93.22% <0%> (-2.09%)	2% <0%> (ø)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...nternal/operators/observable/ObservableWindow.java	98.18% <0%> (-1.82%)	3% <0%> (ø)
...perators/single/SingleFlatMapIterableFlowable.java	95% <0%> (-1.67%)	2% <0%> (ø)
...rnal/operators/observable/ObservableSwitchMap.java	91.57% <0%> (-1.58%)	3% <0%> (ø)
...ternal/operators/completable/CompletableMerge.java	95.94% <0%> (-1.36%)	2% <0%> (ø)
...ivex/rxjava3/subscribers/SerializedSubscriber.java	98.86% <0%> (-1.14%)	26% <0%> (-1%)
...perators/observable/ObservableMergeWithSingle.java	99.05% <0%> (-0.95%)	2% <0%> (ø)
...a3/internal/operators/flowable/FlowableReplay.java	91.97% <0%> (-0.83%)	19% <0%> (ø)
... and 8 more

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