test/stp: pack host structs like SDCC and cover the edge recovery

Build the harness with -fpack-struct=1 and assert the frame struct sizes,
so the host reads wire bytes the way the firmware lays them out. Add
scenarios for a port that regains edge after a hold, one that heard a BPDU
and stays non-edge, auto-edge switched off, and a captured RouterOS BPDU
fed in on every port, tagged and untagged.

test/stp/Makefile

@@ -2,6 +2,7 @@ CC      ?= cc
 SRCROOT  = ../..
 CFLAGS   = -std=c11 -g -O1 -Wall -Wextra -Werror -Wno-unused-parameter -Wno-unknown-pragmas \
            -Wno-pointer-sign \
+           -fpack-struct=1 \
            -DRTLP_HOST_TEST \
            -fsanitize=address,undefined -fno-omit-frame-pointer \
            -include ../sdcc_shim.h -I../stub -I$(SRCROOT) -I$(SRCROOT)/httpd -I$(SRCROOT)/uip