Add fetch_json() test harness API

This allows fetching JSON test data regardless of whether the environment
has fetch() or not; ShadowRealm does not.

In a ShadowRealm environment, this gets the incubating realm to fetch the
data. Every other environment just executes a function that calls fetch().

docs/writing-tests/testharness-api.md

@@ -823,6 +823,29 @@ eventWatcher.wait_for('animationstart').then(t.step_func(function() {
 }));
 ```
 
+### Loading test data from JSON files ###
+
+```eval_rst
+.. js:autofunction:: fetch_json
+```
+
+Loading test data from a JSON file would normally be accomplished by
+something like this:
+
+```js
+promise_test(() => fetch('resources/my_data.json').then((res) => res.json()).then(runTests));
+function runTests(myData) {
+  /// ...
+}
+```
+
+However, `fetch()` is not exposed inside ShadowRealm scopes, so if the
+test is to be run inside a ShadowRealm, use `fetch_json()` instead:
+
+```js
+promise_test(() => fetch_json('resources/my_data.json').then(runTests));
+```
+
 ### Utility Functions ###
 
 ```eval_rst

resources/testharness.js

@@ -4777,6 +4777,15 @@
         return "Untitled";
     }
 
+    /** Fetches a JSON resource and parses it */
+    async function fetch_json(resource) {
+        const response = await fetch(resource);
+        return await response.json();
+    }
+    if (!global_scope.GLOBAL || !global_scope.GLOBAL.isShadowRealm()) {
+        expose(fetch_json, 'fetch_json');
+    }
+
     /**
      * Setup globals
      */

tools/serve/serve.py

@@ -424,6 +424,14 @@ class ShadowRealmHandler(HtmlWrapperHandler):
 (async function() {
   const r = new ShadowRealm();
   r.evaluate("globalThis.self = globalThis; undefined;");
+  r.evaluate(`func => {
+    globalThis.fetch_json = (resource) => {
+      const thenMethod = func(resource);
+      return new Promise((resolve, reject) => thenMethod((s) => resolve(JSON.parse(s)), reject));
+    };
+  }`)((resource) => function (resolve, reject) {
+    fetch(resource).then(res => res.text(), String).then(resolve, reject);
+  });
   await new Promise(r.evaluate(`
     (resolve, reject) => {
       (async () => {