bpf: Tidy xdp attach selftests

Support for cpumap and devmap entry progs in previous commits means the
test needs to be updated for the new semantics. Also take this
opportunity to convert it from CHECK macros to the new ASSERT macros.

Since xdp_cpumap_attach has no subtest, put the sole test inside the
test_xdp_cpumap_attach function.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/bpf/20210702111825.491065-6-memxor@gmail.com