Convert the font-test ttx helper function to use the Fetch API

By replacing `XMLHttpRequest` with a `fetch` call, the helper function can be modernized to use async/await instead.
Note that the headers doesn't seem necessary to set, since:
 - The Fetch API provides a method for accessing the response as *text*, which renders the "Content-type" header unnecessary.
 - According to https://developer.mozilla.org/en-US/docs/Glossary/Forbidden_header_name, the "Content-length" header isn't necessary.

test/font/fontutils.js

@@ -65,26 +65,16 @@ function encodeFontData(data) {
   return buffer;
 }
 
-function ttx(data) {
-  return new Promise((resolve, reject) => {
-    const xhr = new XMLHttpRequest();
-    xhr.open("POST", "/ttx");
-
-    const encodedData = encodeFontData(data);
-    xhr.setRequestHeader("Content-type", "text/plain");
-    xhr.setRequestHeader("Content-length", encodedData.length);
-
-    xhr.onreadystatechange = function getPdfOnreadystatechange(e) {
-      if (xhr.readyState === 4) {
-        if (xhr.status === 200) {
-          resolve(xhr.responseText);
-        } else {
-          reject(new Error(xhr.statusText));
-        }
-      }
-    };
-    xhr.send(encodedData);
+async function ttx(data) {
+  const response = await fetch("/ttx", {
+    method: "POST",
+    body: encodeFontData(data),
   });
+
+  if (!response.ok) {
+    throw new Error(response.statusText);
+  }
+  return response.text();
 }
 
 function verifyTtxOutput(output) {