Added gdb remote tests to verify $Hg{thread-id}.

Added test to check that each thread reported by $q{f,s}ThreadInfo
can be switched to by $Hg, verified by a follow-up $qC.

Modified test exe to accept "thread:new" to create a new thread
that runs and sleeps for 5 seconds.

@llgs_test/@debugserver_test now buffer output.

llgs and debugserver gdbremote protocol tests now collect $O notification
output into the context returned from expect_lldb_gdbserver_replay.
context["O_count"] is an integer indicating the number of $O packets
collected during the replay, and context["O_content"] contains the
accumulated hex-decoded text output by the inferior (stdout and stderr).

Modified the $O check test to check the accumulated output rather than
a direct $O packet.