selftests/bpf: Test BPF_CGROUP_SYSCTL

Add unit test for BPF_PROG_TYPE_CGROUP_SYSCTL program type.

Test that program can allow/deny access.
Test both valid and invalid accesses to ctx->write.

Example of output:
  # ./test_sysctl
  Test case: sysctl wrong attach_type .. [PASS]
  Test case: sysctl:read allow all .. [PASS]
  Test case: sysctl:read deny all .. [PASS]
  Test case: ctx:write sysctl:read read ok .. [PASS]
  Test case: ctx:write sysctl:write read ok .. [PASS]
  Test case: ctx:write sysctl:read write reject .. [PASS]
  Summary: 6 PASSED, 0 FAILED

Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>