Fix tests in test_socket to use correctly CMSG_LEN (GH-9594)

After some failures in AMD64 FreeBSD CURRENT Debug 3.x buildbots
regarding tests in test_socket that are using
testFDPassSeparateMinSpace(), FreeBDS revision 337423 was pointed
out to be the reason the test started to fail.

A close examination of the manpage for cmsg_space(3) reveals that
the number of file descriptors needs to be taken into account when
using CMSG_LEN().

This commit fixes tests in test_socket to use correctly CMSG_LEN, taking
into account the number of FDs.