test: Fix fake StreamingEngine persisting active streams between loads

Player unit tests use a fake StreamingEngine that persists between
loads.  Some tests then load() multiple times, which causes calls to
destroy() on the fake.

But the fake StreamingEngine was not releasing its active stream
references on destroy(), leading to a brittle test that broke when
PR shaka-project#3459 was introduced.

This fixes the fake to release active streams between loads().

Change-Id: Ifea5234863aeb976ddc45c825ef79342a176bc2f

test/test/util/simple_fakes.js

@@ -90,13 +90,15 @@ shaka.test.FakeAbrManager = class {
 /** @extends {shaka.media.StreamingEngine} */
 shaka.test.FakeStreamingEngine = class {
   constructor() {
-    const resolve = () => Promise.resolve();
-
     let activeVariant = null;
     let activeText = null;
 
     /** @type {!jasmine.Spy} */
-    this.destroy = jasmine.createSpy('destroy').and.callFake(resolve);
+    this.destroy = jasmine.createSpy('destroy').and.callFake(() => {
+      activeVariant = null;
+      activeText = null;
+      return Promise.resolve();
+    });
 
     /** @type {!jasmine.Spy} */
     this.configure = jasmine.createSpy('configure');