Update the isc_ht unit test to also tesh rehashing

Update the isc_ht unit test to also tesh rehashing

As incremental rehashing has been added to isc_ht implementation, we
need to test whether the rehashing works.

Update the isc_ht unit test to test:

 * preinitialized hash table large enough to hold all the elements
 * smallest hash table that fully grows to hold all the elements
 * partially preinitialized hash table that grows
 * iterating while rehashing is in progress