fix(core): fix memory leak on application reference destroy

When an application reference is destroyed it stays in memory because it is still registered in the testability registry.

Closes angular#22106, angular#13725

packages/core/src/application_ref.ts

@@ -564,6 +564,10 @@ export class ApplicationRef {
   private _unloadComponent(componentRef: ComponentRef<any>): void {
     this.detachView(componentRef.hostView);
     remove(this.components, componentRef);
+    const testabilityRegistry = componentRef.injector.get(TestabilityRegistry, null);
+    if (testabilityRegistry) {
+      testabilityRegistry.unregisterApplication(componentRef.location.nativeElement);
+    }
   }
 
   /** @internal */