socket-test: don't try to be clever, fail if no XDG_RUNTIME_DIR is set
socket-test: don't try to be clever, fail if no XDG_RUNTIME_DIR is set

Not only setenv(), also putenv() allocates memory on my system
(glibc 2.16.0). Just fail with a clear message if XDG_RUNTIME_DIR is
not set.

https://bugs.freedesktop.org/show_bug.cgi?id=52618

abooij/wayland,sir-murray/wayland,bitfactor/wayland,mchalupa/wayland,tulcod/wayland,bitfactor/wayland,tulcod/wayland,eyolfson/wayland,eyolfson/wayland,sir-murray/wayland,abooij/wayland,sir-murray/wayland,abooij/wayland,tulcod/wayland,mchalupa/wayland,eyolfson/wayland,bitfactor/wayland,mchalupa/wayland