bpf: selftest: Use static globals in tcp_hdr_options and btf_skc_cls_ingress

Some globals in the tcp_hdr_options test and btf_skc_cls_ingress test
are not using static scope.  This patch fixes it.

Targeting bpf-next branch as an improvement since it currently does not
break the build.

Fixes: ad2f8eb0 ("bpf: selftests: Tcp header options")
Fixes: 9a856cae

 ("bpf: selftest: Add test_btf_skc_cls_ingress")
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20201106225402.4135741-1-kafai@fb.com