Show diff when a test fails

Change-Id: Ice5027164e761445a3f25b76832bf183b36de03c

polygerrit-ui/karma.conf.js

diff --git a/polygerrit-ui/karma.conf.js b/polygerrit-ui/karma.conf.js
index 3f7221a..fe3fa0c 100644
--- a/polygerrit-ui/karma.conf.js
+++ b/polygerrit-ui/karma.conf.js
@@ -163,6 +163,9 @@
     // available reporters: https://npmjs.org/browse/keyword/karma-reporter
     reporters: ['mocha'],
 
+    mochaReporter: {
+      showDiff: true
+    },
 
     // web server port
     port: 9876,