2.x: make any() and all() return Single, patch up tests

[akarnokd]

This PR changes the return type of any() and all() to Single.

I've also did a small change to TestObserver by having it implement the other XObserver types. This resulted in one extra method onSuccess to be added but now you can subscribe TestObserver to Single, Maybe and Completable without conversion yet still test() them with the same convenient API.

Flowable.range(1, 10).any(v -> true).test().assertResult(true);

[codecov-io]

Current coverage is 78.61% (diff: 79.69%)

Merging #4573 into 2.x will decrease coverage by 0.10%

@@                2.x      #4573   diff @@
==========================================
  Files           530        534     +4   
  Lines         35337      35518   +181   
  Methods           0          0          
  Messages          0          0          
  Branches       5482       5504    +22   
==========================================
+ Hits          27815      27921   +106   
- Misses         5543       5610    +67   
- Partials       1979       1987     +8   

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