add unit tests

src/app/modules/songs/services/song-list.resolver.spec.ts

@@ -0,0 +1,32 @@
+import {TestBed} from '@angular/core/testing';
+import {of} from 'rxjs';
+import {SongService} from './song.service';
+import {SongListResolver} from './song-list.resolver';
+
+describe('SongListResolver', () => {
+  let resolver: SongListResolver;
+  let songServiceSpy: jasmine.SpyObj<SongService>;
+
+  beforeEach(() => {
+    songServiceSpy = jasmine.createSpyObj<SongService>('SongService', ['list$']);
+    songServiceSpy.list$.and.returnValue(of([{id: 'song-1', title: 'Amazing Grace'}] as never));
+
+    void TestBed.configureTestingModule({
+      providers: [{provide: SongService, useValue: songServiceSpy}],
+    });
+
+    resolver = TestBed.inject(SongListResolver);
+  });
+
+  it('should be created', () => {
+    expect(resolver).toBeTruthy();
+  });
+
+  it('should resolve the first emitted song list from the service', done => {
+    resolver.resolve().subscribe(songs => {
+      expect(songServiceSpy.list$).toHaveBeenCalled();
+      expect(songs).toEqual([{id: 'song-1', title: 'Amazing Grace'}] as never);
+      done();
+    });
+  });
+});