Test the 'networkidle' navigation logic

This patch adds a test to verify that navigation properly waits for the
network to become idle.

References #10

test/assets/networkidle.html

@@ -0,0 +1,5 @@
+<script>
+  fetch('fetch-request-a.js');
+  fetch('fetch-request-b.js');
+  fetch('fetch-request-c.js');
+</script>