Add mocking selectors to MockStore

[nasreddineskandrani]

Following my comment here

I'll open one or multiple PR(s) to update the example-app with MockStore as stated in these comments
#1027 (comment)
I start working on it tomorow.

I added a first component spec update for the example-app in this fork commit.
Based on the current MockStore version and after experiencing it, i can say surely:
the brain time to resolve the mocked state vs just the state needed in the selector is bigger which is a waste of time (especially when you test your selectors independently)

what i did in the commit on top
=> comeback to a simpler case of @maxime1992 comment here

@brandonroberts
i really recommend before updating the doc and the example-app to move forward [now] with your proposal to mock selectors here.

So we can document the MockStore separately in the ngrx doc with the 2 possible methods of mocking and we can use the efficient one for unit testing in the example-app which is the mock selectors.

Note:
A dev asked for it recently
#915 (comment)

If accepted, I would be willing to submit a PR for this feature

[X] Yes (Assistance is provided if you need help submitting a pull request)
[ ] No

I am motivated if you are busy to use your proposal and play with it to make sure it cover all cases... and submit a PR before proceeding with the example-app update.