Make tests pass on Chrome again

It seems that a number of the tests had started failing when run in
Chrome. They were fine under PhantomJS, but the MegolmExport tests only work
under Chrome, and I need them to work...

Mostly the problems were timing-related, where assumptions made about how
quickly the `then` handler on a promise would be called were no longer
valid. Possibly Chrome 55 has made some changes to the relative priorities of
setTimeout and sendMessage calls.

One of the TimelinePanel tests was failing because it was expecting the contents
of a div to take up more room than they actually were. It's possible this is
something very environment-specific; hopefully the new value will work on a
wider range of machines.

Also some logging tweaks.

test/test-utils.js

@@ -14,7 +14,15 @@ var MatrixEvent = jssdk.MatrixEvent;
  */
 export function beforeEach(context) {
     var desc = context.currentTest.fullTitle();
+
     console.log();
+
+    // this puts a mark in the chrome devtools timeline, which can help
+    // figure out what's been going on.
+    if (console.timeStamp) {
+        console.timeStamp(desc);
+    }
+
     console.log(desc);
     console.log(new Array(1 + desc.length).join("="));
 };