tests: add tests for bug in adding socket

Last set of commits introduced a bug. When adding of socket with
a particular name fails, then the socket and its lockfile are deleted
regardless who created the socket.

/* OK */
wl_display_add_socket(display, "wayland-0");

/* this call fails and will delete the original socket */
wl_display_add_socket(display, "wayland-0");

Signed-off-by: Marek Chalupa <mchqwerty@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>