bpf: fix test suite to enable all unpriv program types

Given BPF_PROG_TYPE_CGROUP_SKB program types are also valid in an
unprivileged setting, lets not omit these tests and potentially
have issues fall through the cracks. Make this more obvious by
adding a small test_as_unpriv() helper.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>