Add test that mutation observers receive the default policy value whe…

…n `setAttribute` is called

As requested in
<w3c/trusted-types#425 (comment)>.

trusted-types/block-string-assignment-to-Element-setAttribute.html

@@ -75,6 +75,22 @@
     }, c[0] + "." + c[1] + " accepts string and null after default policy was created.");
   });
 
+  scriptURLTestCases.concat(HTMLTestCases).concat(ScriptTestCases).forEach(c => {
+    async_test(t => {
+      const testElement = document.createElement(c[0]);
+
+      const observer = new MutationObserver(t.step_func_done((aMutations, aObserver) => {
+        assert_equals(aMutations.length, 1);
+        const newValue = aMutations[0].target.getAttribute(c[1]);
+        assert_equals(newValue, c[3]);
+      }));
+
+      observer.observe(testElement, { attributes: true});
+
+      testElement.setAttribute(c[1], c[2]);
+    }, c[0] + "." + c[1] + "'s mutationobservers receive the default policy's value.");
+  });
+
   HTMLTestCases.forEach(c => {
     test(t => {
       assert_element_accepts_trusted_type(c[0], c[1], c[2], c[3]);