selftests: bpf: config: add config fragments

Tests test_tunnel.sh fails due to config fragments ins't enabled.

Fixes: 933a741e ("selftests/bpf: bpf tunnel test.")
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>