Enable slow test reporting

This should help to catch slow tests in development before they become
flakey failures on Travis.

h/static/scripts/karma.config.js

@@ -91,5 +91,8 @@ module.exports = function(config) {
     // Continuous Integration mode
     // if true, Karma captures browsers, runs the tests and exits
     singleRun: false,
+
+    // Log slow tests so we can fix them before they timeout
+    reportSlowerThan: 500,
   });
 };