bpf: selftests: Get rid of CHECK macro in xdp_adjust_tail.c

Rely on ASSERT* macros and get rid of deprecated CHECK ones in
xdp_adjust_tail bpf selftest.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/c0ab002ffa647a20ec9e584214bf0d4373142b54.1642679130.git.lorenzo@kernel.org