avocado.utils.gdb.GDBRemote: introduce utility methods for commonly used commands

[clebergnu]

Some commands, such as s (step), c (continue), and several others are very commonly used. There may be a benefit in having utility methods that employ a common behavior, and return a parsed version of their respective reply packets.

Reference: https://lists.gnu.org/archive/html/qemu-devel/2020-10/msg01435.html