Testing: add basic state store mocking utilities

In order to properly test store-dependent code without the Angular TestBed, some utilities are required that will assist in the creation of a mock store compatible with NgRx. This store will need to be observable, with full observable functionality, as well as be able to issue next values into the observable stream (i.e. be a subject). A basic store interface will be required to maintain compatiblity between both the mock store, as well as the real NgRx store.