Changes required to get abstract working with tape

Tape does not support the following so I have replaced them with
t.type = t.ok(foo instanceof Bar)
t.like = t.ok(comparison logic)
t.isa = t.ok(foo instanceof Bar)

The only oddity is 
https://github.com/No9/node-abstract-leveldown/pull/new/master#L4R126 
Where the it was originally a test for undefined. 
Maybe this should test for both? 
t.ok(err === null, 'err argument is null')