test: Fix exception calling setVolume() in ads test (shaka-project#6731)

MockAdManager did not define setVolume() in this one test, causing the
test to throw an exception. This exception got swallowed and did not
fail the test, but was noticed in the JS console during a Karma debug
run.

test/ads/ad_manager_unit.js

@@ -126,6 +126,10 @@ describe('Ad manager', () => {
             getVolume() {
               return 0;
             }
+
+            setVolume() {
+              // Nothing
+            }
           };
           return new MockAdManager();
         };