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quickly the `then` handler on a promise would be called were no longer
valid.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/components/structures/TimelinePanel-test.js

@@ -99,7 +99,11 @@ describe('TimelinePanel', function() {
         // the document so that we can interact with it properly.
         parentDiv = document.createElement('div');
         parentDiv.style.width = '800px';
-        parentDiv.style.height = '600px';
+
+        // This has to be slightly carefully chosen. We expect to have to do
+        // exactly one pagination to fill it.
+        parentDiv.style.height = '500px';
+
         parentDiv.style.overflow = 'hidden';
         document.body.appendChild(parentDiv);
     });
@@ -235,7 +239,7 @@ describe('TimelinePanel', function() {
                 expect(client.paginateEventTimeline.callCount).toEqual(0);
                 done();
             }, 0);
-        }, 0);
+        }, 10);
     });
 
     it("should let you scroll down to the bottom after you've scrolled up", function(done) {