t3lib_divTest::fixPermissionsSetsGroup fails on MacOS
This test fails on Mac OS because the test makes use of posix_getegid() to find out the current process' group ID. However, this function on Mac OS always returns -1 instead of the actual group ID, making it useless (although it exists).
This test is the only part of the core that makes use of the posix_getegid() function. We should check whether this function returns -1 (which means that we're on Mac OS) and skip the test in that case.