samba3.blackbox.smbclient.forceuser_validusers: Add new test for force user option.

samba3.blackbox.smbclient.forceuser_validusers: Add new test for force user option.

Test covers commit
cf0934caf282f4ade8c8a701b0e40b68a2f17ace

BUG: https://bugzilla.samba.org/show_bug.cgi?id=9878
RH BUG: https://bugzilla.redhat.com/show_bug.cgi?id=1077651

How to test:
$ make -j test TESTS="samba3.blackbox.smbclient.forceuser_validusers"
RESULD: Should PASS
$ git revert cf0934caf282f4ade8c8a701b0e40b68a2f17ace
$ make -j test TESTS="samba3.blackbox.smbclient.forceuser_validusers"
RESULT: Should FAIL

Signed-off-by: Robin Hack <rhack@redhat.com>
Reviewed-by: Uri Simchoni <uri@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
Autobuild-User(master): Uri Simchoni <uri@samba.org>
Autobuild-Date(master): Thu Feb  4 03:44:42 CET 2016 on sn-devel-144