The dns_test_makezone() helper function always assigns the created zone
to some view, which is not always necessary and complicates cleanup of
non-managed zones as they are required not to be assigned to any view.

Rework dns_test_makezone() in order to make it easier to use in unit
tests operating on non-managed zones.  Use dns_name_fromstring() instead
of dns_name_fromtext() to simplify code.  Do not use the CHECK() macro
and add comments to make code flow simpler to follow.  Use
dns_test_makeview() instead of dns_view_create().

Adjust existing unit tests using this function so that they still pass.