Bug 1787008 - Add some dumping of load notifications to a test. r=dholbert

Seems per the logs we're hanging at the beginning of the test which is a
bit surprising. Try to see if we're failing to load any browser in
particular or we're getting an unexpected notification or what not.

Differential Revision: https://phabricator.services.mozilla.com/D172054

layout/base/tests/chrome/test_color_scheme_browser.xhtml

@@ -31,6 +31,7 @@
           let loaded = new Promise(resolve => {
             b.addProgressListener({
               onStateChange(aWebProgress, aRequest, aStateFlags, aStatus) {
+                info(`Got state change for ${b.parentNode.id}: ${aStateFlags}, ${aStatus}`);
                 if (
                   aStateFlags & Ci.nsIWebProgressListener.STATE_IS_WINDOW &&
                   aStateFlags & Ci.nsIWebProgressListener.STATE_STOP