test-compositor: extend leak checks into clients

Run leak checks also on clients of test-compositor.

Checking leaks in memory is the same as in normal TEST.

Checking file descriptor leaks is slightly more complex, as we pass an
open file descriptor in the WAYLAND_SOCKET environment variable, which
will be consumed by a client calling wl_display_connect(), but otherwise
remain unused. We manage this accounting by checking if the environment
variable has been unset by wl_display_connect() as it consumes the
socket.

Signed-off-by: Marek Chalupa <mchqwerty@gmail.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>

[daniels: squashed counter-proposal patch to simplify fd leak checking,
          plus Bill's counter-counter-proposal, and reworded log.]