selftest/bpf: Make xsk tests less verbose

Make the xsk tests less verbose by only printing the
essentials. Currently, it is hard to see if the tests passed or not
due to all the printouts. Move the extra printouts to a verbose
option, if further debugging is needed when a problem arises.

To run the xsk tests with verbose output:
./test_xsk.sh -v

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Link: https://lore.kernel.org/bpf/20210223162304.7450-2-ciara.loftus@intel.com