[FIX] web: correct quarter format in mockReadGroup

Before this commit the value of a field (of date/datetime type)
used as a groupby was not correctly formatted in the test environment
when the granularity was 'quarter'. This commit fixes that situation.

addons/web/static/tests/helpers/mock_server.js

@@ -877,6 +877,8 @@ var MockServer = Class.extend({
                     return moment(val).format('YYYY-MM-DD');
                 } else if (aggregateFunction === 'week') {
                     return moment(val).format('ww YYYY');
+                } else if (aggregateFunction === 'quarter') {
+                    return 'Q' + moment(val).format('Q YYYY');
                 } else if (aggregateFunction === 'year') {
                     return moment(val).format('Y');
                 } else {