gdb: tests: set remotetimeout to gdb_load_timeout for remote targets

gdb: tests: set remotetimeout to gdb_load_timeout for remote targets

Rather than relying on the default remotetimeout value (which might be
too small for some slower devices), use the existing gdb_load_timeout
config option to set it.

This adds two new helpers for getting/setting the remotetimout to keep
the new logic simple.

Signed-off-by: Jie Zhang <jie.zhang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>