tests: Only run test on kernel datapath

tests: Only run test on kernel datapath

The recently added test to check for the correct L3 L4 protocol
information after conntrack reassembles a packet should not run
in the userspace datapath.  It is specific to a kernel datapath
regression.

Also change the name of the test to make it more informative and
less redundant and add comments with a short explanation.

Fixes: d7fd61a ("tests: Add check for correct l3l4 conntrack frag reassembly")
Suggested-by: Darrell Ball <dlu998@gmail.com>
Signed-off-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>