[BUGFIX] Remove PHP Error caused by setlocale call

With #44835 the setlocale function is used to reset manipulations
on locale settings during tests. Accidently the array keys were
stored as strings, but should be constants.

Fixes: #45118
Related: #44835
Releases: 6.1, 6.0, 4.7, 4.6, 4.5
Change-Id: I6514d11b97359778a0ee11ec3115d1cbd0caac75
Reviewed-on: https://review.typo3.org/17987
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters