libphobos, testsuite: Disable forkgc2 on Darwin [PR103944]

It hangs the testsuite (requiring manual intervention to kill the
spawned processes) which breaks CI.  The reason for the hang id not
clear.  This skips the test for now (xfail does not work).

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>

	PR d/103944

libphobos/ChangeLog:

	* testsuite/libphobos.gc/forkgc2.d: Skip for Darwin.