test/stp: tag the frames the simulator feeds in

The frame the simulator builds carries a vlan_tag between the RTL tag and the BPDU, but left its TPID zero. stp_in() now takes the body offset from TRAP_RX_BODY(), which reads those two bytes, so it took the untagged branch and parsed the body four bytes early. Writing 0x8100 makes the frame what its layout says it is.

test/stp/stp_sim.c

@@ -321,6 +321,7 @@ static void bpdu_in(const struct sim_bpdu *b)
 	struct sim_pkt_in *f = (struct sim_pkt_in *)uip_buf;
 
 	memset(uip_buf, 0, sizeof(uip_buf));
+	f->vlan_tag.svlan = HTONS(0x8100);
 	memcpy(f->stp_addr, "\x01\x80\xc2\x00\x00\x00", 6);
 	memcpy(f->src_addr, b->br_mac, 6);
 	f->rtl_tag.pmask = HTONS(b->port);