functestlib: add robust Wayland/Weston helpers, drop headless-path and richer debug

- New helpers: weston_pids, weston_is_running, find_wayland_sockets, wayland_pick_socket,
  adopt_wayland_env_from_socket, wayland_connection_ok, wayland_choose_or_start, wayland_debug_snapshot.
- Socket discovery across /run/user/<uid>, /run/user/*, /dev/socket/weston, /tmp; no hardcoded UID.
- Prefer pgrep; fallback to portable ps+awk (minimal-build friendly).
- Remove headless backend spawn logic, weston_start only tries systemd and otherwise returns non-zero.
- Minimal-build safe: no hard deps (wayland-info/weston-info optional with fallbacks).
- Debug snapshot logs ps/cmdline, socket stat, and reproducible `export` hints.

Signed-off-by: Srikanth Muppandam <smuppand@qti.qualcomm.com>

Author: smuppand