test: Fix exceptions in tests (#4772)

This fixes test exceptions that did not fail their tests. One was an
HlsParser instance that outlived the test run and failed after server
disconnection. The other was an exception in AdManager that did not fail
the test, but nonetheless caused an unexpected error to be logged.

Both of these issues were spotted while running the tests in a local
browser in debug mode. Neither caused test failures directly, but both
were examples of poor hygiene in our tests.

test/ads/ad_manager_unit.js

@@ -115,7 +115,9 @@ describe('Ad manager', () => {
         startedEventB = /** @type {!google.ima.AdEvent} */ (
           new shaka.util.FakeEvent(google.ima.AdEvent.Type.STARTED));
         startedEventB.getAd = () => {
-          return {};
+          return {
+            isLinear: () => true,
+          };
         };
       }
       makeMocks();

test/hls/hls_parser_unit.js

@@ -40,6 +40,7 @@ describe('HlsParser', () => {
 
   afterEach(() => {
     shaka.log.alwaysWarn = originalAlwaysWarn;
+    parser.stop();
   });
 
   beforeEach(() => {