3.x: Reenable XFlatMapTest.maybeSingle, add missing Single operators

[akarnokd]

Reenable the XFlatMapTest.maybeSingle and make sure all flatMap variants behave the same when the flow is cancelled/disposed while the mapper function is executing, thus the returned inner source doesn't get subscribed to at all.

Resolves #6892

While adding the extra tests, I noticed two flatMap variant is missing from Single:

• A combiner of the original and inner success item: flatMap(Function<T, Single<U>>, BiFunction<T, U, R>)
• A notification-type mapper: flatMap(Function<T, Single<R>>, Function<Throwable, Single<R>>)

Lastly, the same combiner variant for Maybe received a marble diagram too:

Related #5806

[codecov]

Codecov Report

Merging #6893 into 3.x will increase coverage by 0.04%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                3.x    #6893      +/-   ##
============================================
+ Coverage     98.25%   98.29%   +0.04%     
- Complexity     6590     6599       +9     
============================================
  Files           740      742       +2     
  Lines         47293    47399     +106     
  Branches       6430     6443      +13     
============================================
+ Hits          46468    46593     +125     
+ Misses          300      292       -8     
+ Partials        525      514      -11

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/rxjava3/core/Maybe.java	100% <ø> (ø)	208 <0> (ø)	⬇️
...3/internal/operators/maybe/MaybeFlatMapSingle.java	100% <100%> (ø)	2 <0> (ø)	⬇️
...rnal/operators/maybe/MaybeFlatMapNotification.java	100% <100%> (ø)	2 <0> (ø)	⬇️
...rc/main/java/io/reactivex/rxjava3/core/Single.java	100% <100%> (ø)	190 <2> (+2)	⬆️
...ernal/operators/single/SingleFlatMapPublisher.java	100% <100%> (ø)	2 <0> (ø)	⬇️
...al/operators/single/SingleFlatMapNotification.java	100% <100%> (ø)	2 <2> (?)
...rnal/operators/single/SingleFlatMapBiSelector.java	100% <100%> (ø)	2 <2> (?)
...ternal/operators/mixed/MaybeFlatMapObservable.java	100% <100%> (ø)	2 <0> (ø)	⬇️
...nternal/operators/mixed/MaybeFlatMapPublisher.java	100% <100%> (ø)	2 <0> (ø)	⬇️
...ernal/operators/mixed/SingleFlatMapObservable.java	100% <100%> (ø)	2 <0> (ø)	⬇️
... and 28 more

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