[test] Persist packets between expect_gdbremote_sequence invocations

Summary:
Some tests (Hc_then_Csignal_signals_correct_thread, at least) were sending a "continue" packet in
one expect_gdbremote_sequence invocation, and "expecting" the stop-reply in another call. This
posed a problem, because the were packets were not persisted between the two invocations, and if
the stub was exceptionally fast to respond, the packet would be received in the first invocation
(where it would be ignored) and then the second invocation would fail because it could not find
the packet.

Since doing matching in two invocations seems like a reasonable use of the packet pump, instead
of fixing the test, I make sure the packet_pump supports this usage by making the list of
captured packets persistent.

Reviewers: tfiala

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D18140

llvm-svn: 263629