rrule_manager_test is failing again! (Part 3) - Backport for 3.1

Note https://superuser.com/questions/983905/linux-date-command-not-working Set your PC's date to a month where it's the 2nd day of the month and the first Monday of the month as well. (e.g. 02-01-2017, 02-10-2017) Run the core_calendar_rrule_manager_testcase: vendor/bin/phpunit calendar/tests/rrule_manager_test.php Confirm that everything passes. Set your PC's date to a month where the first Monday is after the 2nd day of the month. (e.g. 06-02-2017, 03-04-2017) Run the core_calendar_rrule_manager_testcase: vendor/bin/phpunit calendar/tests/rrule_manager_test.php Confirm that everything passes. Set your PC's date to a month where the first Monday is before the 2nd day of the month. (e.g. 01-05-2017, 01-01-2018) Run the core_calendar_rrule_manager_testcase: vendor/bin/phpunit calendar/tests/rrule_manager_test.php Confirm that everything passes. After the test: Make sure to set your system date back to how it was.