[TASK] Stabilize scheduler tests and activate in travis

Next try to add the scheduler tests to the test suites.

The patch applies several changes:

* For some weird reasons PHP does not recognize 'September' as valid
month keyword if the timezone is not Europe/Berlin. The test is removed
since only three letter abbreviations are allowed anyway.

* Make the cron command test system timezone agnostic.
CronCommandTest now explicitly sets the timezone to UTC (GMT) in setUp
and has a backup for tearDown. This is needed because the tests use hard
coded timestamps and the system under test is sensitive for timezones.
This was the main reason for the travis runs to fail in the first place.

* The dataProvider in CronCommandTest do not execute strtotime anymore.
The reason for that is, that the data provider data in phpunit is
executed before setUp and won't get the timezone setting then. The main
data provider is now splitted into two, separating the strtotime()
calculation off from the rest.

Change-Id: Ifb8de929b8e6edb31b24cbf02a9ddaa9dda9ab43
Resolves: #40515
Releases: 6.0
Reviewed-on: http://review.typo3.org/14549
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn