2.x: cleanup newline separation, some field namings

[akarnokd]

This PR cleans up some source code style inconsistencies:

• Rename some atomic-subscription fields to upstream (left out from 2.x: Cleaunp - rename fields to upstream and downstream #6129).
• Make sure @Test methods are separated by at most one empty newline
• Make sure a closing curly brace and an annotation is only followed by one empty newline.
• Add a test that checks for the above.

[codecov]

Codecov Report

Merging #6150 into 2.x will increase coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##               2.x    #6150      +/-   ##
===========================================
+ Coverage     98.2%   98.23%   +0.02%     
- Complexity    6197     6199       +2     
===========================================
  Files          667      667              
  Lines        44857    44857              
  Branches      6213     6213              
===========================================
+ Hits         44054    44064      +10     
+ Misses         260      253       -7     
+ Partials       543      540       -3

Impacted Files	Coverage Δ	Complexity Δ
...nternal/operators/observable/ObservableReplay.java	98.63% <ø> (ø)	23 <0> (ø)	⬇️
.../operators/observable/ObservableOnErrorReturn.java	100% <ø> (ø)	2 <0> (ø)	⬇️
...nternal/operators/single/SingleInternalHelper.java	100% <ø> (ø)	5 <0> (ø)	⬇️
...internal/operators/observable/ObservableCount.java	100% <ø> (ø)	2 <0> (ø)	⬇️
...rnal/operators/flowable/FlowableDematerialize.java	100% <ø> (ø)	2 <0> (ø)	⬇️
...ernal/operators/observable/ObservableSkipLast.java	100% <ø> (ø)	2 <0> (ø)	⬇️
...internal/operators/flowable/FlowableConcatMap.java	97.74% <ø> (-0.38%)	6 <0> (ø)
.../operators/mixed/CompletableAndThenObservable.java	100% <ø> (ø)	2 <0> (ø)	⬇️
...al/operators/observable/ObservableSampleTimed.java	100% <ø> (ø)	3 <0> (ø)	⬇️
...rnal/operators/observable/ObservableSkipWhile.java	100% <ø> (ø)	2 <0> (ø)	⬇️
... and 121 more

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