System tests: Add 4 new ftp and tftp tests.

System tests: Add 4 new ftp and tftp tests.

In order to have full coverage of ALGs for the userspace
datapath, it is necessary to add 4 new tests. Three of these will
cover passive ftp, including basic V6 passive ftp, V4 passive ftp
with NAT and sequence skew and V6 passive ftp with NAT. The last
test will cover tftp with NAT. Before these additions, there was
only one part of one test covering passive ftp without NAT and
one basic tftp test without NAT.

Signed-off-by: Darrell Ball <dlu998@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>