test: factor throwaway wltest clients into _run_wl_client (phase 4)
9 uniform throwaway-client sites (late/tag, third tab, globals
prober, layer bars x2, foreign requester, takeover locker, modifiers
and popup-reposition oneshots) go from Popen + try/finally
terminate/wait to with _run_wl_client(sock, *args, **kw):. Deliberately
untouched: host-side wl-copy (host DISPLAY env), frame/menu X11 pairs
and popup a/b (shared/explicit mid-body lifecycles), the probe (custom
DISPLAY + log-file stdio), and the _oneshot_*/_poll_* helpers.
Full suite: 152 passed, 2 skipped.
Andrew Hamilton
f10fff910c