selftests/bpf: Make netcnt selftests serial to avoid spurious failures

When running `./test_progs -j` test_netcnt fails with a very high
probability, undercounting number of packets received (9999 vs expected
10000). It seems to be conflicting with other cgroup/skb selftests. So
make it serial for now to make parallel mode more robust.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20211103054113.2130582-1-andrii@kernel.org