Motivation:

Behavior of `Single.concat(Publisher)` is different compare to all other
`concat` variants: `Single.concat(Completable)`,
`Single.concat(Single)`, `Completable.concat(Completable),
`Completable.concat(Single)`, `Completable.concat(Publisher)`,
`Publisher.concat(Completable)`, `Publisher.concat(Single)`,
`Publisher.concat(Publisher)`. It does not subscribe to the next source
to propagate cancellation if `onSuccess` is delivered after `cancel`.

Modifications:

- Make tests for all `concat` valiants consistent in regards to
cancellation;
- Modify `Single.concat(Publisher)` behavior to subscribe in case
`onSuccess` is delivered after `cancel`;

Result:

Behavior of `Single.concat(Publisher)` is consistent with all other
`concat` variants.

…lisher