fix(cdk/testing): TestbedHarnessEnvironment should work when Zone is …

…not present

This commit fixes the harness environment which assumes Zone is always
defined on the page.

src/cdk/testing/testbed/testbed-harness-environment.ts

@@ -69,7 +69,7 @@ function uninstallAutoChangeDetectionStatusHandler(fixture: ComponentFixture<unk
 
 /** Whether we are currently in the fake async zone. */
 function isInFakeAsyncZone() {
-  return Zone!.current.get('FakeAsyncTestZoneSpec') != null;
+  return typeof Zone !== 'undefined' || Zone!.current.get('FakeAsyncTestZoneSpec') != null;
 }
 
 /**