[BUGFIX] Skip a SelectViewHelperTest on Mac

The optionsAreSortedByLabelIfSortByOptionLabelIsSetAndLocaleEqualsUtf8
test always fails on Mac OSX caused by a bug in the C libraries on
BSD/OSX. Skip the test if PHP_OS is Darwin.

Fixes: #45291
Releases: 6.1, 6.0, 4.7, 4.6, 4.5

Change-Id: I79087ec91a2da60d0e5c44bb0fedfaf11f3508bc
Reviewed-on: https://review.typo3.org/18133
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn