tests: Ensure that overflow test always overflows

While the default Unix socket buffer size on Linux is relatively
small, on some computers the default size may be configured to
be huge, making the overflow test never actually overflow the
Wayland display socket.

The changed code now explicitly sets the display socket send buffer
size to be small enough to guarantee an overflow.

Signed-off-by: Manuel Stoeckl <code@mstoeckl.com>