Store: MockReducerManager crashes with reducerManager.addFeatures is not a function

[eddietisma]

Minimal reproduction of the bug/regression with instructions:

provideMockStore crashes when using forFeature imports.

1. Add a module that imports using ngrx forFeature.

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

2. Writing a unit test with provideMockStore and import the module.

TypeError: reducerManager.addFeatures is not a function

Stackblitz with repro:
https://stackblitz.com/edit/ngrx-seed-wrbv4t

Expected behavior:

provideMockStore should support feature modules.

platform/modules/store/testing/src/mock_reducer_manager.ts has no-op for addFeatures.

Versions of NgRx, Angular, Node, affected browser(s) and operating system(s):

"@angular-devkit/core": "8.0.1",
"@angular-devkit/schematics": "8.0.1",
"@angular/common": "8.0.0",
"@angular/compiler": "8.0.0",
"@angular/core": "8.0.0",
"@angular/forms": "8.0.0",
"@angular/platform-browser": "8.0.0",
"@angular/platform-browser-dynamic": "8.0.0",
"@angular/router": "8.0.0",
"@ngrx/effects": "8.0.0-rc.0",
"@ngrx/entity": "8.0.0-rc.0",
"@ngrx/router-store": "8.0.0-rc.0",
"@ngrx/schematics": "8.0.0-rc.0",
"@ngrx/store": "8.0.0-rc.0",
"@ngrx/store-devtools": "8.0.0-rc.0",

Other information:

I would be willing to submit a PR to fix this issue

[x] Yes (Assistance is provided if you need help submitting a pull request)
[ ] No