test: refactor index.html to include mocha setup as an external script

This was done so the sister webpack build can use the same mocha setup script

tests/index.html

@@ -11,14 +11,7 @@
   <script src="../node_modules/chai/chai.js"></script>
   <script src="../node_modules/mocha/mocha.js"></script>
 
-  <script>
-    mocha.setup('bdd').timeout(10000).slow(250);
-
-    window.onload = function() {
-      mocha.checkLeaks();
-      mocha.run();
-    };
-  </script>
+  <script src="./setup-mocha.js"></script>
 
   <!-- Add any new tests to `tests/index.js` -->
   <script src="./index.js"></script>

tests/setup-mocha.js

@@ -0,0 +1,8 @@
+var mocha = require('mocha');
+
+mocha.setup('bdd').timeout(10000).slow(250);
+
+window.onload = function() {
+    mocha.checkLeaks();
+    mocha.run();
+};