feat(store): add API to mock selectors

[brandonroberts]

Closes #1504

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngrx/platform/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Closes #1504

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

Adds a MockStore#overrideSelector(string | selector) method that short-circuits returned values from selectors. Covers mocking different types of selectors:

store.select('slice');

const selector = createSelector(state, state => state.slice);
store.select(selector);

const selector = createSelector(state, state => state.slice);
store.pipe(select(selector));

[ngrxbot]

Preview docs changes for 2d9f8ac at https://previews.ngrx.io/pr1688-2d9f8ac/

[nasreddineskandrani]

i'll try it this week! amazing! thank you very much. I am introducing ngrx to a company and this will be really helpful!

[timdeschryver]

Neat! Just a few nits

[ngrxbot]

Preview docs changes for 558b132 at https://previews.ngrx.io/pr1688-558b132/

[ngrxbot]

Preview docs changes for e9f13fe at https://previews.ngrx.io/pr1688-e9f13fe/

[timdeschryver]

👍