fixed ua tests were triggering unneeded events

@@ -4,12 +4,21 @@ function testUA(ua, match) {
     );
 }
 
-testUA(casper.options.pageSettings.userAgent, /CasperJS/);
-
-casper.start().userAgent('plop').on('resource.requested', function(request) {
+function fetchUA(request) {
     testUA(request.headers.filter(function(header) {
         return header.name === "User-Agent";
     }).pop().value, /plop/);
-}).start('tests/site/index.html').run(function() {
+}
+
+testUA(casper.options.pageSettings.userAgent, /CasperJS/);
+
+casper.start();
+
+casper.userAgent('plop').on('resource.requested', fetchUA);
+
+casper.thenOpen('tests/site/index.html');
+
+casper.run(function() {
+    this.removeListener('resource.requested', fetchUA);
     this.test.done();
 });