Move frontend tests out of zerver/tests/.

This fixes an unfortunate bug where the backend tests in
zerver/tests.py were not being run automatically, and also makes these
a bit easier to find.

frontend_tests/casperjs/tests/commands/script.js

@@ -0,0 +1,3 @@
+var casper = require('casper').create();
+casper.echo('it works');
+casper.exit();