Fix timelinepanel test timeout

The previous fix was adjusting the wrong timeout: it's hitting browserNoActivityTimeout, so revert previous fix & do this instead

karma.conf.js

@@ -171,5 +171,6 @@ module.exports = function (config) {
             },
             devtool: 'inline-source-map',
         },
+        browserNoActivityTimeout: 15000,
     });
 };

test/components/structures/TimelinePanel-test.js

@@ -210,7 +210,7 @@ describe('TimelinePanel', function() {
         var N_EVENTS = 600;
 
         // sadly, loading all those events takes a while
-        this.timeout(N_EVENTS * 40);
+        this.timeout(N_EVENTS * 20);
 
         // client.getRoom is called a /lot/ in this test, so replace
         // sinon's spy with a fast noop.