Ensures month has changed when we click to change month.

This avoid mandelbug:
https://travis-ci.org/collective/collective.cover/jobs/416408698#L1740

src/collective/cover/tests/test_calendar_tile.robot

@@ -13,7 +13,6 @@ ${calendar_tile_location}  'collective.cover.calendar'
 *** Test cases ***
 
 Test Calendar Tile
-    [Tags]  Mandelbug
 
     Enable Autologin as  Site Administrator
     Go to Homepage
@@ -40,9 +39,11 @@ Test Calendar Tile
     ...    return $next.attr('data-month');
     ...  })();
     Wait Until Keyword Succeeds  5 sec  1 sec  Click Link  css=a.calendar-tile-next
+    Wait Until Page Does Not Contain Element  xpath=.//a[@class='calendar-tile-next'][@data-month='${nextmonth}']
     Wait Until Keyword Succeeds  5 sec  1 sec  Click Link  css=a.calendar-tile-next
     Wait Until Page Contains Element  xpath=.//a[@class='calendar-tile-prev'][@data-month='${nextmonth}'][@data-year='${nextyear}']
     Wait Until Keyword Succeeds  5 sec  1 sec  Click Link  css=a.calendar-tile-prev
+    Wait Until Page Does Not Contain Element  xpath=.//a[@class='calendar-tile-prev'][@data-month='${nextmonth}']
     Wait Until Keyword Succeeds  5 sec  1 sec  Click Link  css=a.calendar-tile-prev
     Wait Until Page Contains Element  xpath=.//a[@class='calendar-tile-next'][@data-month='${nextmonth}'][@data-year='${nextyear}']