tests/hash: extract test out of xf86drmHash.c

This way with follow up commits we can fix it and wire it up to
make check

v2:
 - Use xf86drmHash.h for common structs.(Jan)
 - Add test to .gitignore.

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