feat(TestScheduler): Add subscription schedule to expectObservable

[gstamac]

Description:
I made this PR (#3429) already for the previous version but was rejected because of API changes. Now the new API is here and I'm making the same PR.

Related issue (if exists): #3429

[coveralls]

Coverage increased (+0.2%) to 96.948% when pulling 01faee6 on gstamac:master into 37c5c09 on ReactiveX:master.

[benlesh]

Could you please describe the use cases and motivation for the change? This isn't anything that has been considered or discussed on the team, so my immediate concern is that there isn't a need for it. Giving us concrete examples of where it's a useful feature will help us decide what we should do here.

[gstamac]

An example use would be to test shareReplay.
The test

it('should multicast the same values to multiple observers, bufferSize=1', () => {
    const source =     cold('-1-2-3----4-|'); const shared = source.pipe(shareReplay(1));
    const sourceSubs =      '^           !';
    const subscriber1 = hot('a|           ').pipe(mergeMapTo(shared));
    const expected1   =     '-1-2-3----4-|';
    const subscriber2 = hot('    b|       ').pipe(mergeMapTo(shared));
    const expected2   =     '    23----4-|';
    const subscriber3 = hot('        c|   ').pipe(mergeMapTo(shared));
    const expected3   =     '        3-4-|';

    expectObservable(subscriber1).toBe(expected1);
    expectObservable(subscriber2).toBe(expected2);
    expectObservable(subscriber3).toBe(expected3);
    expectSubscriptions(source.subscriptions).toBe(sourceSubs);
});

would be rewritten to something like this

it('should multicast the same values to multiple observers, bufferSize=1', () => {
    const source =      cold('-1-2-3----4-|'); const shared = source.pipe(shareReplay(1));
    const sourceSubs =       '^           !';
    const expected1   =      '-1-2-3----4-|';
    const expected2   =      '    23----4-|';
    const expected3   =      '        3-4-|';

    expectObservable(shared, '^           !').toBe(expected1);
    expectObservable(shared, '    ^       !').toBe(expected2);
    expectObservable(shared, '        ^   !').toBe(expected3);
    expectSubscriptions(source.subscriptions).toBe(sourceSubs);
});

[benlesh]

@gstamac That's a great example! I'll add this to our core team agenda so we can discuss it. Thanks!

[benlesh]

#4006 fixes the travis issue.

[benlesh]

Thanks, @gstamac! Good addition!