Moodle release 2.5.3
MDL-42276 core_group: added behat tests to automate group actions

1. Added new step definitions to check if an element exists or does not exist in another.
2. Added new step definitions to check if an element is readonly or is not readonly.
3. Automated testing for MDLQA-1812 and MDLQA-1813.

Added behat test for deleting groups/groupings with and without the
'moodle/course:changeidnumber' capability.
4. Automated MDLQA-2105.

Added behat test for creating groups/groupings without the
'moodle/course:changeidnumber' capability.
5. Automated MDLQA-2238 and MDLQA-2335.

Added behat test for updating groups/groupings with and without the
'moodle/course:changeidnumber' capability.