Common testing module

Now that we have a lot of modules to deal with, it becomes tedious to configure the imports and providers for each spec file individually. For this purpose, I recommend creating a common testing module to contain generic configuration that you can reuse across the board.

First start by creating a new .ts file

1. Create common/common.testing.ts.
2. Populate it with common testing providers, fakes, and modules.

   I have provided fake implementations of ObservableMedia, MatIconRegistry, and DomSanitizer, along with arrays for commonTestingProviders and commonTestingModules:

   src/app/common/common.testing.ts
   import { 
     HttpClientTestingModule 
   } from '@angular/common/http/ testing'
   import { SecurityContext } from '@angular/core'
   import { MediaChange } from '@angular/flex-layout'
   import { ReactiveFormsModule } from '@angular/forms'
   import { 
     SafeResourceUrl, 
     SafeValue 
   } from '@angular/platform...