bpf/selftests: Test that kernel rejects a TCP CC with an invalid license

This adds a selftest to check that the verifier rejects a TCP CC struct_ops
with a non-GPL license.

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20210326100314.121853-2-toke@redhat.com