Add tests for fragment() / defragment()

As suggested by @guedou, this adds tests for fragment() and
defragment() functions.

test/regression.uts

@@ -4435,3 +4435,32 @@ assert pkt.dst == pkt.real_dst
 assert pkt.src == pkt.real_src
 assert pkt.tos == 0
 assert pkt.function == 0
+
+
+############
+############
++ Test fragment() / defragment() functions
+
+= fragment()
+payloadlen, fragsize = 100, 8
+assert fragsize % 8 == 0
+fragcount = (payloadlen / fragsize) + bool(payloadlen % fragsize)
+* create the packet
+pkt = IP() / ("X" * payloadlen)
+* create the fragments
+frags = fragment(pkt, fragsize)
+* count the fragments
+assert len(frags) == fragcount
+* each fragment except the last one should have MF set
+assert all(p.flags == 1 for p in frags[:-1])
+assert frags[-1].flags == 0
+* each fragment except the last one should have a payload of fragsize bytes
+assert all(len(p.payload) == 8 for p in frags[:-1])
+assert len(frags[-1].payload) == ((payloadlen % fragsize) or fragsize)
+
+= defragment()
+defrags = defragment(frags)
+* we should have one single packet
+assert len(defrags) == 1
+* which should be the same as pkt reconstructed
+assert defrags[0] == IP(str(pkt))