functestlib: improve weston overlay helpers and process detection

Make the Weston/Wayland overlay helpers more robust for the weston-simple-egl
test and similar graphics workloads.

- Rework overlay_start_weston_drm() to:
  - Use /dev/socket/weston as a private XDG_RUNTIME_DIR.
  - Create and chmod the runtime dir safely.
  - Start Weston with --continue-without-input and an explicit log file.
  - Poll for the Wayland socket up to a fixed timeout and return non-zero if
    no socket appears.
  - Avoid leaving stray Weston processes running when startup fails.

- Add and wire up Weston helpers used by the Graphics suite:
  - discover_wayland_socket_anywhere() to locate existing Wayland sockets.
  - adopt_wayland_env_from_socket() to set XDG_RUNTIME_DIR and WAYLAND_DISPLAY
    based on a discovered socket.
  - wayland_connection_ok() to sanity-check Wayland connectivity using
    wayland-info when available.
  - weston_pick_env_or_start() to prefer existing Weston sessions and only
    spawn a new one when needed, with detailed logging and socket dumps.

- Ensure the overlay path plays nicely with the base path by logging the
  suggested env exports so manual reproduction is straightforward.

Also fix ShellCheck SC2009 in is_process_running() by:

- Preferring pgrep -x <name> for name-based checks when pgrep is present.
- Falling back to a ps -e | awk pipeline in minimal environments without pgrep.
- Keeping numeric-input behaviour (kill -0 on PID) intact.
- Preserving existing log messages and 0/1 return codes so current callers
  continue to work unchanged.

Signed-off-by: Srikanth Muppandam <[email protected]>

Author: smuppand