It would help testing of heavily used templates if there was an option that a group of users testing the new template could turn on in their preferences, so that the wikipedia software would check the user space specified in this option FIRST for any template called, then look in the regular template area.
This would ease testing of templates, in that a user could turn this option on, and any page that was displayed would call in the test template instead of the "public" template. With this option, a group of users would not have to have a group of "dummied" up examples to test - they could easily test a heavily used template for days, and more easily find problems, because they would be easily be able to test the template on a wide variety of existing Wikipedia pages without changing any of those pages.
The way the software pulled in templates would only be affected for those users with this option on. No one else would see the tested template. Then when the testing group was satisfied, they would agree to have the template moved to the main template area and they would turn off the testing option in their preferences.
This is something that is commonly done in other systems for the same reason - to allow a group of users to test new programs/templates. It would be equivalent to having certain users call in a "test concatenation" to test macros in assembler language, or defining a testing path with test libraries in front for Perl (http://perldoc.perl.org/lib.html)