bpf/selftests: convert xdp_link test to ASSERT_* macros

Convert the selftest to use the preferred ASSERT_* macros instead of the
deprecated CHECK().

v2:
- Don't add if statements around checks if they weren't there before.

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/r/20220107221115.326171-2-toke@redhat.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>