Note that comment and gen-class return nil in :unused-ret-vals docs

Also in unused-ret-vals lint messages where the value is nil.  It is
not possible, as far as I can tell, to distinguish a nil in the source
code with a comment or gen-class form in the source code, from looking
at the ast.
Update suspicious-test linter so it avoids some incorrect warnings

In particular, it no longer warns if the 2nd arg to is is a form like
(str ...) or (format ...), because those should evaluate to strings.
Make output shorter if there are only a few differences in unit test failures