Force allocating a PTY when running gdbserver

A gdbserver may be running left over from a previous debug session. It will still be listening on the TCP port, preventing a new instance from starting, so we need to kill it.

However, if we allocate a PTY when spawning it, it will die once the session ends.

For some reason, with get_pty=True, the gdbserver’s output is coming from stdout, not stderr.

This will fix T4463.

Signed-off-by: Andrej Shadura <andrew.shadura@collabora.co.uk>