2.x: Fix Flowable.flatMap not canceling the inner sources on outer error

[akarnokd]

The outer onError did not cancel the inner sources. The Observable variant works correctly but both received an unit test to verify the behavior.

Fixes: #6825

[codecov]

Codecov Report

Merging #6827 into 2.x will increase coverage by 0.05%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #6827      +/-   ##
============================================
+ Coverage     98.24%   98.29%   +0.05%     
- Complexity     6348     6350       +2     
============================================
  Files           677      677              
  Lines         45532    45535       +3     
  Branches       6334     6336       +2     
============================================
+ Hits          44733    44759      +26     
+ Misses          253      244       -9     
+ Partials        546      532      -14

Impacted Files	Coverage Δ	Complexity Δ
...x/internal/operators/flowable/FlowableFlatMap.java	89.55% <100%> (-0.45%)	4 <0> (ø)
...nternal/operators/observable/ObservableCreate.java	89.74% <0%> (-7.7%)	2% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	86.92% <0%> (-2.62%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	96.25% <0%> (-1.61%)	5% <0%> (ø)
...perators/observable/ObservableMergeWithSingle.java	99.06% <0%> (-0.94%)	2% <0%> (ø)
...internal/operators/flowable/FlowableSwitchMap.java	94.9% <0%> (-0.47%)	3% <0%> (ø)
...ex/internal/operators/flowable/FlowableCreate.java	97.09% <0%> (ø)	6% <0%> (ø)	⬇️
...main/java/io/reactivex/subjects/ReplaySubject.java	97.51% <0%> (+0.41%)	50% <0%> (ø)	⬇️
...perators/mixed/ObservableConcatMapCompletable.java	100% <0%> (+0.75%)	3% <0%> (ø)	⬇️
... and 14 more

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[davidmoten]

looks good, thanks, important fix!