3.x: add marbles to X.fromSupplier

[akarnokd]

This PR adds marbles to the new fromSupplier operators:

Flowable:

Observable:

Maybe:

Single:

Completable:

[codecov]

Codecov Report

Merging #6531 into 3.x will decrease coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #6531      +/-   ##
============================================
- Coverage     98.23%   98.23%   -0.01%     
- Complexity     6188     6189       +1     
============================================
  Files           680      680              
  Lines         44883    44883              
  Branches       6193     6193              
============================================
- Hits          44092    44090       -2     
- Misses          248      255       +7     
+ Partials        543      538       -5

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Flowable.java	100% <ø> (ø)	566 <0> (ø)	⬇️
src/main/java/io/reactivex/Completable.java	100% <ø> (ø)	118 <0> (ø)	⬇️
src/main/java/io/reactivex/Observable.java	100% <ø> (ø)	541 <0> (ø)	⬇️
src/main/java/io/reactivex/Maybe.java	100% <ø> (ø)	172 <0> (ø)	⬇️
src/main/java/io/reactivex/Single.java	100% <ø> (ø)	148 <0> (ø)	⬇️
...ernal/operators/flowable/FlowableFlatMapMaybe.java	88.88% <0%> (-6.77%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.93% <0%> (-2.72%)	2% <0%> (ø)
...activex/internal/observers/QueueDrainObserver.java	97.43% <0%> (-2.57%)	21% <0%> (-1%)
...tivex/internal/operators/single/SingleTimeout.java	98.33% <0%> (-1.67%)	2% <0%> (ø)
...rnal/operators/observable/ObservableSwitchMap.java	94.68% <0%> (-1.6%)	3% <0%> (ø)
... and 23 more

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