fix: add missing test MutationObserver

Author: mhevery

Diff for: test/browser_entry_point.ts

@@ -12,10 +12,11 @@ import './browser/browser.spec';
 import './browser/define-property.spec';
 import './browser/element.spec';
 import './browser/FileReader.spec';
+// import './browser/geolocation.spec.manual';
 import './browser/HTMLImports.spec';
+import './browser/MutationObserver.spec';
 import './browser/registerElement.spec';
 import './browser/requestAnimationFrame.spec';
 import './browser/WebSocket.spec';
 import './browser/XMLHttpRequest.spec';
-// import './browser/geolocation.spec.manual';
 import './mocha-patch.spec';