2.x: Add Create test with CancelAble for all types and add more MaybeCreateTest cases

[vanniktech]

setCancellable was not tested before.

[vanniktech]

Gradle suite > Gradle test > io.reactivex.tck.ObserveOnTckTest. FAILED
java.lang.AssertionError: Illegal concurrent access detected (entering critical section)! Thread[RxSingleScheduler-1,5,main] emited onNext(0) signal, before Thread[Test worker,5,main] finished its onSubscribe() signal.

[akarnokd]

TCK doesn't seem to like that onSubscribe may trigger emission before it even returns - which is completely legal btw. I'll add a workaround for that in the TCK shortly.

[codecov-io]

Current coverage is 78.60% (diff: 100%)

Merging #4540 into 2.x will decrease coverage by 0.02%

@@                2.x      #4540   diff @@
==========================================
  Files           530        530          
  Lines         35334      35334          
  Methods           0          0          
  Messages          0          0          
  Branches       5482       5482          
==========================================
- Hits          27782      27773     -9   
- Misses         5576       5584     +8   
- Partials       1976       1977     +1   

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