tests/fake_snap: sanitize paths

tests/fake_snap: sanitize paths

Ensure fake_snap.pl can be run in taint mode (-T), by sanitizing paths
and the PATH env. This fixes the following samba3.rpc.fsrvp selftest
failures:
Insecure dependency in mkdir while running setgid at (eval 2) line 4.
snap create failed: NT_STATUS_UNSUCCESSFUL
snap create failed for shadow copy of /home/ddiss/isms/samba/st/nt4_dc/share

Bug: https://bugzilla.samba.org/show_bug.cgi?id=12988

Signed-off-by: David Disseldorp <ddiss@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Tue Aug 29 04:54:51 CEST 2017 on sn-devel-144

(cherry picked from commit f9d4158f0b002b482df0a919d4cb337cce81f9f8)

Autobuild-User(v4-6-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-6-test): Thu Aug 31 16:14:33 CEST 2017 on sn-devel-144