tests/hash: misc compilation fixes

Get the test from completely broken to working like a charm.

 - Use the same variable type for both HashInsert and HashLookup.
 - Use correct storage type for the HashLookup return value.
 - Remove useless backward iteration of HashLookup(i).

v2:
 - Use void * instead of unsigned long.
 - Change value to key << 16 | key.

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