test: add subscribeWith tests

spec/util/subscribeWith-spec.ts

@@ -0,0 +1,18 @@
+import { expect } from 'chai';
+import { of, Subscriber } from 'rxjs';
+import { subscribeWith } from 'rxjs/internal/util/subscribeWith';
+import { asInteropObservable, asInteropSubscriber } from '../helpers/interop-helper';
+
+describe('subscribeToResult', () => {
+  it('should return the subscriber for interop observables', () => {
+    const subscriber = new Subscriber<number>();
+    const subscription = subscribeWith(asInteropObservable(of(42)), subscriber);
+    expect(subscription).to.equal(subscriber);
+  });
+
+  it('should return the subscriber for interop subscribers', () => {
+    const subscriber = asInteropSubscriber(new Subscriber<number>());
+    const subscription = subscribeWith(of(42), subscriber);
+    expect(subscription).to.equal(subscriber);
+  });
+});