2.x: Add missing Completable marbles (+19, 07/19a)

[akarnokd]

This PR add even more missing marbles to Completable (tracked by #5789):

delay

doOnX

doFinally

onErrorComplete

onErrorResumeNext

onTerminateDetach

mergeWith

concatWith

observeOn

hide

[codecov]

Codecov Report

Merging #6097 into 2.x will decrease coverage by 0.06%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##               2.x    #6097      +/-   ##
===========================================
- Coverage     98.3%   98.23%   -0.07%     
- Complexity    6196     6199       +3     
===========================================
  Files          667      667              
  Lines        44862    44862              
  Branches      6213     6213              
===========================================
- Hits         44102    44071      -31     
- Misses         231      242      +11     
- Partials       529      549      +20

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Completable.java	100% <ø> (ø)	117 <0> (ø)	⬇️
...l/operators/observable/ObservableFlatMapMaybe.java	84.96% <0%> (-9.81%)	2% <0%> (ø)
.../operators/observable/ObservableFlatMapSingle.java	88.8% <0%> (-4.48%)	2% <0%> (ø)
...nternal/operators/observable/ObservableCreate.java	95.72% <0%> (-2.57%)	2% <0%> (ø)
...x/internal/operators/flowable/FlowablePublish.java	97.44% <0%> (-1.28%)	11% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	94.11% <0%> (-1.07%)	5% <0%> (ø)
.../operators/mixed/FlowableSwitchMapCompletable.java	98.94% <0%> (-1.06%)	2% <0%> (ø)
...ava/io/reactivex/processors/BehaviorProcessor.java	98.2% <0%> (-0.9%)	59% <0%> (-1%)
...operators/observable/ObservableMergeWithMaybe.java	99.1% <0%> (-0.9%)	2% <0%> (ø)
...l/operators/observable/ObservableTimeoutTimed.java	99.15% <0%> (-0.85%)	3% <0%> (ø)
... and 14 more

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