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.

Apertis: T4463

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