fix: chain subscriptions from interop observables

[cartant]

Description:

This PR fixes a bug in which subscriptions returned from interop observables are ignored - instead of being composed into the subscription chain. Prior to this fix, when an interop observable was returned to a flattening operator - like mergeMap or switchMap - the interop observable would not be unsubscribed upon explicit unsubscription from the flattening operator.

The bug was introduced in the changes made in #2479 and #4037.

This PR:

• adds a test helper for simulating an interop observable;
• copies existing, chaining-related tests and uses an interop observable within them;
• ensures subscriptions returned from interop observables are added to the subscription chain;
• fixes another bug in which subscribeToResult was passed outer values and indices when they should have been passed to the InnerSubscriber (the bug would have manifested itself in the now-deprecated result selectors for the flattening operators); and
• tweaks the subscribeToResult signature to make it safer (if an InnerSubscriber is passed, value and index must be undefined and must be passed via the InnerSubscriber instead).

Related issue (if exists): #5051

[benlesh]

This looks good to me... However, I'd love it if we could add comments to explain why things are structured the way the are in the changed areas. Also, a brief comment in the test about what an "interop" is, just explain the scenario. :)

Thanks, @cartant!

[cartant]

Added comments.

[benlesh]

@cartant: Unfortunately, the patch branch will need a separate PR. Can you tackle that? The problem is that the tests have changed between the two branches.