selftests: xsk: make the stats tests normal tests

Make the stats tests look and feel just like normal tests instead of
bunched under the umbrella of TEST_STATS. This means we will always
run each of them even if one fails. Also gets rid of some special case
code.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/r/20220510115604.8717-9-magnus.karlsson@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>