bpf: selftests: Get rid of CHECK macro in xdp_bpf2bpf.c

Rely on ASSERT* macros and get rid of deprecated CHECK ones in
xdp_bpf2bpf bpf selftest.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/df7e5098465016e27d91f2c69a376a35d63a7621.1642679130.git.lorenzo@kernel.org