Switch to karma-mocha-reporter in tests

This produces more helpful output when test assertions involving deep
equality checks fail.

See https://www.npmjs.com/package/karma-mocha-reporter#showdiff

This requires karma-mocha >= v0.2.2

h/static/scripts/karma.config.js

@@ -63,10 +63,17 @@ module.exports = function(config) {
       }
     },
 
-    // test results reporter to use
-    // possible values: 'dots', 'progress'
-    // available reporters: https://npmjs.org/browse/keyword/karma-reporter
-    reporters: ['dots'],
+    mochaReporter: {
+      // Display a helpful diff when comparing complex objects
+      // See https://www.npmjs.com/package/karma-mocha-reporter#showdiff
+      showDiff: true,
+      // Only show the total test counts and details for failed tests
+      output: 'minimal',
+    },
+
+    // Use https://www.npmjs.com/package/karma-mocha-reporter
+    // for more helpful rendering of test failures
+    reporters: ['mocha'],
 
     // web server port
     port: 9876,

package.json

@@ -75,6 +75,7 @@
     "karma-browserify": "^5.0.3",
     "karma-chai": "^0.1.0",
     "karma-mocha": "^0.2.2",
+    "karma-mocha-reporter": "^2.0.0",
     "karma-phantomjs-launcher": "^0.2.3",
     "karma-sinon": "^1.0.4",
     "mocha": "^2.4.5",