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 <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

tools/testing/selftests/bpf/prog_tests/netcnt.c

@@ -8,7 +8,7 @@
 
 #define CG_NAME "/netcnt"
 
-void test_netcnt(void)
+void serial_test_netcnt(void)
 {
 	union percpu_net_cnt *percpu_netcnt = NULL;
 	struct bpf_cgroup_storage_key key;