[TASK] Clean up API of tx_scheduler_CronCmd_Normalize

The only public API method in tx_scheduler_CronCmd_Normalize is normalize().
The patch makes all non API methods protected and uses an accessible proxy
class in unit tests to make the low level methods testable. This cleans
up the API and still gives detailed unit tests.

Change-Id: I6845d049523b68e6f57d86ad03ef0a48c453df81
Resolves: #29100
Reviewed-on: http://review.typo3.org/4421
Reviewed-by: Björn Pedersen
Reviewed-by: Susanne Moog
Tested-by: Susanne Moog
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn