tests/random: extract test out of xf86drmRandom.c

With follow up commits we can clear it up and wire to
make check

v2:
 - Use xf86drmRandom.h for common struct.(Jan)
 - Add test to .gitignore.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Jan Vesely <jan.vesely@rutgers.edu>