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

--HG--
branch : ICMPv6NDOptDNSSL fixes

--HG--
branch : Support for DNS Search List Option in RA

--HG--
branch : Patch for PR#115

--HG--
branch : Patch for PR#82

--HG--
branch : issue-5105

--HG--
branch : issue-5105

--HG--
branch : UTS fixes

Issues #884 & #915: IPv6 NA is not a NS & Error in class ICMPv6ND_NS(_ICMPv6NDGuessPayload, _ICMPv6, Packet):

Packet.build() is called only on the first layer. It will call
Packet.do_build() which will recurse through all layers. For each layer,
it will call Packet.self_build() which will do the job for each layer.
Packet.do_build() will call Packet.post_build() and transform on them,
and concatenate them. Then Packet.build() will call Packet.build_padding()
and Packet.build_done().

Note that this undetermined case will have a different behavior:
Raw("ABC")/Padding("abc")/Raw("DEF")/Padding("def")

It was "ABCabcDEFdef"
Now it is "ABCDEFabcdef"

A deprecation warning will be issued before complete removal.