conntrack: fix expectations for ftp+DNAT.

conntrack: fix expectations for ftp+DNAT.

When configuring the nat part of an expectation, care must be taken to
look at the master nat action and direction to properly reproduce it.

DNAT tests have been added to both active and passive modes, all
ftp/tftp tests titles have been updated to reflect they are dealing with
SNAT.

Fixes: bd5e81a0e596 ("Userspace Datapath: Add ALG infra and FTP.")
Co-authored-by: Darrell Ball <dlu998@gmail.com>
Signed-off-by: Darrell Ball <dlu998@gmail.com>
Signed-off-by: David Marchand <david.marchand@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>