Improve test driver performance.

Upgrading Underscore to 1.4.4 fixes a bug where _.throttle would sometimes take
double its timeout to deliver calls. So with this bugfix, our test suite
performs worse because reactivity happens more often. Reduce the throttling to
one update of the big table per second instead of twice a second. (Also, there's
no need to defer and call flush; the changed call does that!)

packages/test-in-browser/driver.js

@@ -397,7 +397,7 @@ var reportResults = function(results) {
     countDeps.changed();
   }
 
-  _.defer(_throttled_update);
+  _throttled_update();
 };
 
 // forget all of the events for a particular test
@@ -417,5 +417,4 @@ var forgetEvents = function (results) {
 
 var _throttled_update = _.throttle(function() {
   resultsDeps.changed();
-  Deps.flush();
-}, 500);
+}, 1000);