tests: Add check for correct l3l4 conntrack frag reassembly

tests: Add check for correct l3l4 conntrack frag reassembly

Two commits recently fixed an issue with setting the corrrect l3 and l4
flow information when conntrack reassembles packet fragments.

c98f776 datapath: Clear the L4 portion of the key for "later" fragments
2609173 datapath: Properly set L4 keys on "later" IP fragments

This test checks for regressions that might break this feature.  It
counts on the fact that when the bug is present the udp src port
will not be correct.  It will either be zero or else some other
garbage value.  So the test feeds some fragments through for
reassembly and then checks to make sure that the udp srce port
is actually the correct value of 5001.

Tested by reverting the above commits and observing that the test
then fails.

Signed-off-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>