selftests, bpf: Extend test_tc_tunnel test with vxlan

Add BPF_F_ADJ_ROOM_ENCAP_L2_ETH flag to the existing tests which
encapsulates the ethernet as the inner l2 header.

Update a vxlan encapsulation test case.

Signed-off-by: Xuesen Huang <huangxuesen@kuaishou.com>
Signed-off-by: Li Wang <wangli09@kuaishou.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/bpf/20210305123347.15311-1-hxseverything@gmail.com