Moved a "tests" import inside the calling function for py2exe.

Specifically, unless you do this, you have to individually
import every single test manually whenever you want to compile
with py2exe, since py2exe doesn't support lazy imports.

You probably still have to do the manual includes if you're
actually using the DialogWrapper.RunTests() function, but this
way at least you don't have to muck about with the stuff when
you're not using any tests functionality at all.