Test for the behavior of then()

There needed to be test for the then() callback returning a promise or
throwing an error.
Also added test for the optional callbacks in Y.when()
Remove getResult(), unnecessary test in _notify

Resolver#getResult was unnecesesary since we're not exposing it to the
promise and is not necessary for internal use.
A check for the existance of "subs" in _notify was a left over from the
previous version in which some callback arrays could be gone. Now that
array always exists.