…ngModule

Tests sometimes do not mock out the `PlatformLocation` and end up
affecting real browser state. This can mean changing the real URL of the
browser during a test, updating the History state object, or any number
of other stateful operations. This can result in a test unintentionally affecting
other tests in the suite because the browser state does not usually get
reset before the next test runs. Providing `MockPlatformLocation` by
default prevents these types of accidental test leakages.

In addition, not providing `MockPlatformLocation` by default led to
developers needing to add `RouterTestingModule` to their test suite to
avoid the problems above. This module has spy `Location` providers which
prevent those issues. This commit now makes `RouterTestingModule`
obsolete. Developers can now just use `RouterModule.forRoot` or
`provideRouter` directly in tests _without_ needing to learn to import
additional test providers or modules.

With this, we should consider deprecating `RouterTestingModule` altogether and
migrating developers to `RouterModule.forRoot` or `provideRouter` instead. There
are some small differences between `SpyLocation` and
`MockPlatformLocation` that might cause tests to fail after the
migration (`MockPlatformLocation` is actually more correct in its
behaviors). If this happens, we can advise developers to also add
`provideLocationMocks()` to their test providers, which would re-provide
the `SpyLocation` like before and should make the tests pass again.

BREAKING CHANGE: `MockPlatformLocation` is now provided by default in tests.
Existing tests may have behaviors which rely on
`BrowserPlatformLocation` instead. For example, direct access to the
`window.history` in either the test or the component rather than going
through the Angular APIs (`Location.getState()`). The quickest fix is to
update the providers in the test suite to override the provider again
`TestBed.configureTestingModule({providers: [{provide: PlatformLocation, useClass: BrowserPlatformLocation}]})`.
The ideal fix would be to update the code to instead be compatible with
`MockPlatformLocation` instead.