feat(component-store): add a testing library

[BobobUnicorn]

PR Checklist

Please check if your PR fulfills the following requirements:

• [x] The commit message follows our guidelines: https://github.com/ngrx/platform/blob/master/CONTRIBUTING.md#commit
• [x] 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?

There is no simple unified way that NgRx provides to mock a ComponentStore; this adds a mocking library with a simple API to facilitate writing tests for components that use the ComponentStore.

Closes #2767.

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

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[markostanimirovic]

I will close this PR as it has not been updated for a long time.