selftest: apply NSS_WRAPPER_HOSTNAME to child processes

selftest: apply NSS_WRAPPER_HOSTNAME to child processes

Currently, Samba3.pm returns a value for NSS_WRAPPER_HOSTNAME in provision, but
selftest.pl does not apply it, so Samba3.pm /also/ sets it in its own
environment. This breaks a command like this:

make test TESTS="samba3.blackbox.smbclient_ntlm.plain samba3.rpc.samba3.netlogon"

... since samba3.blackbox.smbclient_ntlm.plain runs in an nt4_member env,
thereby setting ENV{NSS_WRAPPER_HOSTNAME} to the value for a member, and
samba3.rpc.samba3.netlogon depended on NSS_WRAPPER_HOSTNAME as a username (until
previous commit).

Signed-off-by: Jamie McClymont <jamiemcclymont@catalyst.net.nz>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>