feat(store): add option to mock selectors in MockStoreConfig

[jtcrowson]

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 #1827

What is the new behavior?

Adds a mock selectors option to the MockStoreConfig. This will enable developers to mock selectors up front in the testing module configuration, similar to how you can provide an initial state.

This enables developers who do not need to update the state or selector to omit creating a MockStore instance.

describe('Auth Guard', () => {
  let guard: AuthGuard;
  // let store: MockStore<fromAuth.State>;
  // don't need local instance

  beforeEach(() => {
    TestBed.configureTestingModule({
      providers: [
        AuthGuard,
        provideMockStore({
          initialState: //...,
          selectors: [{ selector: fromAuth.getLoggedIn, value: false }]
          // able to provide multiple selectors without local instance
        }),
      ],
    });
    guard = TestBed.get(AuthGuard);
  });

  it('should return false if the user state is not logged in', () => {
    const expected = cold('(a|)', { a: false });
    expect(guard.canActivate()).toBeObservable(expected);
  });
});

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

Should I add documentation in this PR or a follow up PR? Seems to usually be done as a follow up.

[ngrxbot]

Preview docs changes for e009b2f at https://previews.ngrx.io/pr1836-e009b2f/

[timdeschryver]

LGTM, left some minor comments.

[timdeschryver]

Any reason why this if statement is here? I think the overrideSelector is the same 😃

[jtcrowson]

Typescript doesn't seem to be able to infer between the two:

[timdeschryver]

What if we would tweak the overrideSelector type?

overrideSelector<
    T,
    Result,
    SelectorKind =
      | string
      | MemoizedSelector<T, Result>
      | MemoizedSelectorWithProps<T, any, Result>
>(selector: SelectorKind, value: Result): SelectorKind;

[jtcrowson]

Tweaking the overrideSelector type straighten things out :

However, it breaks the type checking of overrideSelector in the wild:

Should have a type error on the last line because false is not an applicable value for the selector.

[ngrxbot]

Preview docs changes for ed7cc49 at https://previews.ngrx.io/pr1836-ed7cc49/