Basically, testing your templates and your application shoud be two separate things. You should use your app-testing-templates (core-templates) to test your actual templates, ie designer-processed-templates (production-templates).

• First do a syntaxt check (making sure there are no template language errors).
• Then check all the required template elements are present, that is, if the a core-template looks like

  [% IF FOO %]
  blah blah [% FOO | html %]
  [% END %]
  blah blah blah
  [download]

  then the production-template must also contain an "IF FOO" block inside which the variable "FOO" is filled in, and filtered through "html".

So, I envision somebody writing Template::Test::Structure ( which comes with the commandline frontend tstest ), which would make sure template-new is structurally equivalent to template-original, warning the user of any differences, as in

 tstest new original
 new:34: variable FOO doesn't use FILTER `html' like original:44: in b
+lock "IF FOO"
 new: missing variable BAR like original:66
[download]

This is all the template designer should be interested in (a structurally valid template), unless, like merlyn said, they also need to make sure the template is valid xml, which they can go ahead and test by running the application test suite or whatever...

See also Unit Testing Generated HTML