Fix GSMTAP linktype + type/sub-type values so Wireshark decodes PCAP

pcap.py: LINKTYPE_GSMTAP (155) was wrong — 155 is DLT_USER8, so
Wireshark showed 'USER 8 / DLT=155'. Now write Ethernet-encapsulated
PCAP (LINKTYPE 1): each GSMTAP message wrapped in Ethernet + IPv4 +
UDP (port 4729), with correct checksums — matching a real tcpdump
capture of GSMTAP-over-UDP and how Wireshark's gsmtap dissector is keyed.

gsmtap.py: GSMTAP_TYPE_SIM was 0x07 (WiMAX burst) — corrected to 0x04;
GSMTAP_SIM_APDU was 0x02 (PPS req) — corrected to 0x00; added PPS_REQ/
PPS_RSP. These match libosmocore/Wireshark packet-gsmtap.h.

Verified with tshark: PCAP now decodes as GSM SIM (SELECT/STATUS/ATR)
instead of 'USER 8'.

tests: pcap tests assert Ethernet linktype + framing + GSMTAP payload.

simtrace2_pysniff/gsmtap.py

@@ -8,10 +8,12 @@ import socket
 
 GSMTAP_VERSION = 0x02
 GSMTAP_HDR_LEN = 4  # in 32-bit words (16 bytes)
-GSMTAP_TYPE_SIM = 0x07
+GSMTAP_TYPE_SIM = 0x04
 
+GSMTAP_SIM_APDU = 0x00
 GSMTAP_SIM_ATR = 0x01
-GSMTAP_SIM_APDU = 0x02
+GSMTAP_SIM_PPS_REQ = 0x02
+GSMTAP_SIM_PPS_RSP = 0x03
 
 GSMTAP_UDP_PORT = 4729