Prevent using fetch or ReadableStream in tests

Until they can be mocked, they can't be used

ui/app/utils/fetch.js

@@ -1,6 +1,8 @@
+import Ember from 'ember';
 import fetch from 'fetch';
 
 // The ember-fetch polyfill does not provide streaming
-const fetchToUse = window.fetch || fetch;
+// Additionally, Mirage/Pretender does not support fetch
+const fetchToUse = Ember.testing ? fetch : window.fetch || fetch;
 
 export default fetchToUse;

ui/tests/index.html

@@ -21,6 +21,11 @@
     {{content-for "body"}}
     {{content-for "test-body"}}
 
+    <script>
+      // Spoof a lack of ReadableStream support to use XHRs instead of Fetch
+      window.__ReadableStream = window.ReadableStream;
+      window.ReadableStream = undefined;
+    </script>
     <script src="/testem.js" integrity=""></script>
     <script src="{{rootURL}}assets/vendor.js"></script>
     <script src="{{rootURL}}assets/test-support.js"></script>