mock_gdb_server: rectify ack handling code

The mock server was sending acks back in response to spurious acks from
the client, but the client was not prepared to handle these. Most of the
time this would work because the only time the client was sending
unsolicited acks is after the initial connection, and there reply-ack
would get ignored in the "flush all packets from the server" loop which
came after the ack. However, this loop had only a 10ms delay, and
sometimes this was not enough to catch the reply (which meant the
connection got out of sync, and test failed).

Since this behavior not consistent with how lldb-server handles this
situation (it just ignores the ack), I fix the mock server to do the
same.

llvm-svn: 323953