Some NSS modules like those for sssd and LDAP might allocate fds
on a one-per-process basis, for example a socket to talk to sssd.
Make sure those have already been allocated before we enter the code
under test, so that they don't show up as having been "leaked" by the
first module of code under test that happens to do a NSS lookup.

The call to _dbus_test_check_memleaks tears down libdbus' own memory
allocations, but not any hidden state in libc or NSS.

Signed-off-by: Simon McVittie <smcv@collabora.com>