tests/krb5: Add tests for the primary group

tests/krb5: Add tests for the primary group

Primary groups are handled differently from normal groups of which a
user is simply a member. Of particular note is the case where a
domain-local group is made a primary group; a case normally disallowed
by Windows, but not by Samba. Therefore we want tests for it.

Our testing framework must be able to set the user's primary group, and
to clean up afterwards; to set the primary group RID of a PAC; and to
check that the primary group RID is as expected in the PAC returned to
us.

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>