stop prefixing our embedded subtests with "[subtest]"

...as that kinda looks silly with the new Test::More comments.
initial add: t/validate_attribute/in_roles.t
_attribute_options_ok: correct TB::Level reporting

Off-by-one errors are my second least favourite type of error.
nit! doc
add _find_attribute(), a helper function

...because roles have a limited way of looking up attributes.
has_attribute_ok: use _find_attribute()

Yes, this is a *slight* change in behaviour.  Find it, then complain -- I
suspect this isn't a big deal, but I'm open to corrections. :)
validate_attribute: use _find_attribute
attribute_options_ok: use _find_attribute
_attribute_options_ok: skip role attributes

...because those are going to require a significantly different approach.