modetest: Use threads for cursors instead of SIGALRM

This fixes an issue when trying to use -v and -C together. When trying
to read the page flip event, we are interrupted by the SIGALRM that
comes in, and so we think we timed out when we simply got EINTR. While
we could just loop checking for EINTR, SIGALRM is just bad idea to
begin with, so just rewrite it to use a thread.

Signed-off-by: Rob Clark <robclark@freedesktop.org>