Add class_ok() and object_ok() to t/test.pl.

Add class_ok() and object_ok() to t/test.pl.

Change every existing instance of isa_ok() to use object_ok().  This is safe because
before this point, t/test.pl's isa_ok() only worked on objects.  lib/dbmt_common.pl is
the last hold out because it uses Test::More.

These are like isa_ok() but they also check if it's a class or an object.
This lets the core tests defend against outlandish bugs while allowing
t/test.pl to retain feature parity with Test::More.