fix(store): added noop for addFeature in MockReducerManager

[eddietisma]

PR Checklist

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• 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?

[x] Bugfix
[ ] 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?

It is currently not possible to use provideMockStore if the component has a dependency to a module which uses forFeature imports.

StoreModule.forFeature('feature', FeatureReducer),
EffectsModule.forFeature([FeatureEffects]),

Currently crashes with:
TypeError: reducerManager.addFeatures is not a function

Closes #2263

What is the new behavior?

Added noop for addFeature.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[ngrxbot]

Preview docs changes for 82bb972 at https://previews.ngrx.io/pr2265-82bb972/

[brandonroberts]

Thanks @eddietisma!