selftests: xsk: Make enums lower case

Make enums lower case as that is the standard. Also drop the
unnecessary TEST_MODE_UNCONFIGURED mode.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210825093722.10219-16-magnus.karlsson@gmail.com