selftests/bpf: fail test_tunnel.sh if subtests fail

Right now test_tunnel.sh always exits with success even if some
of the subtests fail. Since the output is very verbose, it's
hard to spot the issues with subtests. Let's fail the script
if any subtest fails.

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>