2.x: Add marbles for Observable (12/06)

[akarnokd]

This PR adds some missing marbles to Observable:

• concatMapCompletable
• concatMapDelayError
• concatMapEager
• concatMapEagerDelayError
• concatMapIterable
• doAfterNext
• doFinally

Tracked in #5319, images in comment.

[codecov]

Codecov Report

Merging #5755 into 2.x will decrease coverage by 0.07%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5755      +/-   ##
============================================
- Coverage     96.31%   96.23%   -0.08%     
+ Complexity     5840     5838       -2     
============================================
  Files           634      634              
  Lines         41650    41650              
  Branches       5769     5769              
============================================
- Hits          40115    40083      -32     
- Misses          612      625      +13     
- Partials        923      942      +19

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	509 <0> (ø)	⬇️
.../operators/completable/CompletableConcatArray.java	93.75% <0%> (-6.25%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.93% <0%> (-3.81%)	2% <0%> (ø)
...ternal/operators/observable/ObservablePublish.java	92.98% <0%> (-3.51%)	11% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	86.27% <0%> (-3.27%)	2% <0%> (ø)
...a/io/reactivex/internal/util/QueueDrainHelper.java	58.86% <0%> (-2.84%)	31% <0%> (-2%)
.../internal/operators/flowable/FlowableInterval.java	94.44% <0%> (-2.78%)	3% <0%> (ø)
...ex/internal/subscriptions/SubscriptionArbiter.java	97.45% <0%> (-2.55%)	47% <0%> (-1%)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	94.2% <0%> (-2.42%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableSkipLastTimed.java	95.91% <0%> (-2.05%)	2% <0%> (ø)
... and 25 more

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