Add marbles for Single.from operators

[UMFsimke]

Here are marbles for fromCallable, fromPublisher and fromObservable operators from #5788

Marble for fromCallable

Marble for fromPublisher

Marble for fromObservable

[codecov]

Codecov Report

Merging #6087 into 2.x will decrease coverage by 0.06%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #6087      +/-   ##
============================================
- Coverage     98.25%   98.18%   -0.07%     
+ Complexity     6197     6192       -5     
============================================
  Files           667      667              
  Lines         44858    44858              
  Branches       6211     6211              
============================================
- Hits          44076    44046      -30     
- Misses          239      259      +20     
- Partials        543      553      +10

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Single.java	100% <100%> (ø)	146 <1> (ø)	⬇️
...ex/internal/operators/flowable/FlowableCreate.java	90% <0%> (-7.1%)	6% <0%> (ø)
...tivex/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
...ava/io/reactivex/processors/BehaviorProcessor.java	95.96% <0%> (-3.14%)	59% <0%> (-1%)
...nternal/operators/observable/ObservableCreate.java	95.72% <0%> (-2.57%)	2% <0%> (ø)
.../io/reactivex/internal/schedulers/IoScheduler.java	89.24% <0%> (-2.16%)	9% <0%> (ø)
...rnal/operators/flowable/FlowableSkipLastTimed.java	95.91% <0%> (-2.05%)	2% <0%> (ø)
.../io/reactivex/disposables/CompositeDisposable.java	98.14% <0%> (-1.86%)	39% <0%> (-1%)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.93% <0%> (-1.64%)	2% <0%> (ø)
...rnal/operators/observable/ObservableSwitchMap.java	94.68% <0%> (-1.6%)	3% <0%> (ø)
... and 13 more

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