[infra] Correct integration test

By default, "single-page tests" must include an explicit call to the
global `done` function. Insert this invocation to signal completion and
avoid a test timeout when the test's expectation is satisfied.

infrastructure/browsers/firefox/prefs.html

@@ -3,5 +3,6 @@
 <script src="/resources/testharnessreport.js"></script>
 <script>
 assert_equals(getComputedStyle(document.documentElement).color, "rgb(0, 255, 0)")
+done();
 </script>
 <p>This should be green</p>