Set Mocha to retry failed tests 3 times

no issue

- we occasionally see random errors which fail tests
- this is a problem because it's blocking us seeing which tests are
  really failing
- for now, retry the tests 3 times to overcome the intermittent problem
  until a better solution is found

Gruntfile.js

@@ -117,6 +117,7 @@ const configureGrunt = function (grunt) {
                 timeout: '30000',
                 save: grunt.option('reporter-output'),
                 require: ['core/server/overrides'],
+                retries: '3',
                 exit: true
             },