gdbstub: add test for untimely stop-reply packets

gdbstub: add test for untimely stop-reply packets

In the previous commit, we modified gdbstub.c to only send stop-reply
packets as a response to GDB commands that accept it. Now, let's add a
test for this intended behavior. Running this test before the fix from
the previous commit fails as QEMU sends a stop-reply packet
asynchronously, when GDB was in fact waiting an ACK.

Signed-off-by: Matheus Tavares Bernardino <quic_mathbern@quicinc.com>
Acked-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Taylor Simpson <tsimpson@quicinc.com>
Message-Id: <a30d93b9a8d66e9d9294354cfa2fc3af35f00202.1683214375.git.quic_mathbern@quicinc.com>