[Reproducers] Fix flakiness and off-by-one during replay.

This fixes two replay issues that caused the tests to behave
erratically:

1. It fixes an off-by-one error, where all replies where shifted by 1
   because of a `+` packet that should've been ignored.

2. It fixes another off-by-one-error, where an asynchronous ^C was
   offsetting all subsequent packets. The reason is that we
   'synchronize' requests and replies. In reality, a stop reply is only
   sent when the process halt. During replay however, we instantly
   report the stop, as the reply to packets like continue (vCont).

Both packets should be ignored, and indeed, checking the gdb-remote log,
no unexpected packets are received anymore.

Additionally, be more pedantic when it comes to unexpected packets and
return an failure form the replay server. This way we should be able to
catch these things faster in the future.

llvm-svn: 364494