refactor(TestScheduler): return hot and cold observables

[mpodlasin]

Description:

Return HotObservable and ColdObservable instead of simply Observable, in order to give users access to subscriptions parameter.

Motivation:

Previously getting subscriptions of observables for expectSubscriptions assertions was impossible without typecasting:

const hot = scheduler.createHotObservable('--a---b--|');
hot.subscriptions // does not compile

[coveralls]

Coverage remained the same at 97.698% when pulling 79f2f9e on Podlas29:access-to-subscriptions into 31cf2bf on ReactiveX:master.

[kwonoj]

Looks good to me.

[lock]

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