fix arena, add fetch test

src/tests/fetch/fetch.html

@@ -0,0 +1,16 @@
+<script src="../testing.js"></script>
+<script id=fetch type=module>
+  const promise1 = new Promise((resolve) => {
+    fetch('http://127.0.0.1:9582/xhr/json')
+    .then((res) => {
+      return res.json()
+    })
+    .then((json) => {
+      resolve(json);
+    });
+  });
+
+  testing.async(promise1, (json) => {
+    testing.expectEqual({over: '9000!!!'}, json);
+  });
+</script>

src/tests/fetch/response.html

@@ -36,3 +36,14 @@
   let emptyResponse = new Response("");
   testing.expectEqual(200, emptyResponse.status);
 </script>
+
+<script id=json type=module>
+  const promise1 = new Promise((resolve) => {
+    let response = new Response('[]');
+    response.json().then(resolve)
+  });
+
+  testing.async(promise1, (json) => {
+    testing.expectEqual([], json);
+  });
+</script>ref