This happens for the mkdirSetsGroupOwnershipOfCreatedDirectory() test in
t3lib_divTest.

Solution, just skip the test if the function does not exists.

Also do same safety check for posix_getegid() as done some tests above.

Change-Id: I18cbebc0b22ae5a7318673ed8abd2bd01003ba57
Fixes: #33718
Releases: 6.0, 4.7
Reviewed-on: http://review.typo3.org/8867
Reviewed-by: Tolleiv Nietsch
Tested-by: Tolleiv Nietsch
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert