test: Don't fail tests when cast property can't be deleted (#7342)

On some real Cast devices, you can't delete window.cast. This is fine,
and shouldn't cause the test to fail. If this happens, skip the test
instead. The test, which mocks everything including the platform
detection and the Cast API, is still valid on platforms where we can get
away with those mocks.

test/polyfill/media_capabilities_unit.js

@@ -183,7 +183,15 @@ describe('MediaCapabilities', () => {
         async () => {
           // Temporarily remove window.cast to trigger error. It's restored
           // after every test.
-          delete window['cast'];
+          try {
+            delete window['cast'];
+            // eslint-disable-next-line no-restricted-syntax
+          } catch (error) {
+            // This may fail on some Cast devices where you can't delete the
+            // cast API.  The test should still be valid on non-Cast devices,
+            // since we're mocking everything including the platform.
+            pending('Unable to delete window.cast');
+          }
 
           const isChromecastSpy =
               spyOn(shaka['util']['Platform'],