2.x: Several more Completable marbles (7/19b)

[akarnokd]

This PR adds a couple of more marble diagrams to Completable (#5789):

subscribeOn

unsubscribeOn

toSingle, toSingleDefault

toFlowable

toMaybe

[codecov]

Codecov Report

Merging #6098 into 2.x will decrease coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #6098      +/-   ##
============================================
- Coverage     98.27%   98.25%   -0.03%     
- Complexity     6194     6197       +3     
============================================
  Files           667      667              
  Lines         44862    44862              
  Branches       6213     6213              
============================================
- Hits          44087    44078       -9     
- Misses          232      234       +2     
- Partials        543      550       +7

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Completable.java	100% <ø> (ø)	117 <0> (ø)	⬇️
...l/operators/observable/ObservableFlatMapMaybe.java	89.54% <0%> (-5.89%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	90.33% <0%> (-3.87%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	94.11% <0%> (-3.21%)	5% <0%> (ø)
.../io/reactivex/internal/schedulers/IoScheduler.java	89.24% <0%> (-2.16%)	9% <0%> (ø)
...ternal/operators/completable/CompletableMerge.java	96.42% <0%> (-1.2%)	2% <0%> (ø)
...perators/mixed/ObservableSwitchMapCompletable.java	98.94% <0%> (-1.06%)	3% <0%> (ø)
...ernal/operators/flowable/FlowableTimeoutTimed.java	98.37% <0%> (-0.82%)	3% <0%> (ø)
...a/io/reactivex/internal/util/QueueDrainHelper.java	97.22% <0%> (-0.7%)	56% <0%> (ø)
...vex/internal/operators/flowable/FlowableCache.java	97.94% <0%> (-0.69%)	10% <0%> (-1%)
... and 20 more

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