Added a packet-playback test facility for lldb-gdbserver and a few starter tests.

lldbgdbserverutils.py has a new expect_lldb_gdbserver_replay() method
that plays back gdb remote send/receive packets. These packets are the
log lines that come from running the 'log enable gdb-remote packets',
either from the lldb-gdbserver side or the lldb side. There's a flag
to flip which side is the send (lldb-gdbserver input or lldb-gdbserver
output).

This first checkin tests the initial gdbremote handshake, the ability
to turn on the no-ack mode communication style, thread suffix support,
and list threads in stop reply support. The last two are marked xfail
as top of tree does not yet support these.

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M    test/tools/lldb-gdbserver/TestLldbGdbServer.py
M    test/tools/lldb-gdbserver/lldbgdbserverutils.py