samba3.py: Correctly initialize cache directory for passdb test

samba3.py: Correctly initialize cache directory for passdb test

Running 'make test TESTS=tests.samba3' succeeds, but the log shows that
it tried to open the gencache tdb in the wrong directory:
Unable to create directory /usr/local/samba/var/cache for file gencache.tdb. Error was No such file or directory

Fix this by correctly initializing the cache directory.

Signed-off-by: Christof Schmitt <cs@samba.org>
Reviewed-By: Jelmer Vernooij <jelmer@samba.org>
Autobuild-User(master): Christof Schmitt <cs@samba.org>
Autobuild-Date(master): Fri Jan 16 02:36:39 CET 2015 on sn-devel-104