feat(store): testing - clean up mock store a bit

[alex-okrushko]

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?

• Add tests that would consistently catch resetSelectors in afterEach
• Make selectors private, instead of static
• cleanup constructor
• remove function overloads for overrideSelector - can we typed all at once

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[x] 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 #

What is the new behavior?

Does this PR introduce a breaking change?

MockStore.selectors is not longer static or visible. I don't think that's the API that we meant to support to begin with.

[x] Yes
[ ] No

Other information

[ngrxbot]

Preview docs changes for 43f58a7 at https://previews.ngrx.io/pr2361-43f58a7/

[brandonroberts]

@alex-okrushko will you take a look at why the example app tests are failing? I'd like to land this before we cut another beta

[alex-okrushko]

@alex-okrushko will you take a look at why the example app tests are failing? I'd like to land this before we cut another beta

Yes. Looking into it right now