I don't feel the need to actually install the tests, fetching them from a fresh CPAN archive (with the same version number) sounds good enough to me. Just like in the CPAN shell, you can do test Foo::Bar even when you have the latest version of this module installed, except that it should skip the build phase.

BTW, I think it's unfortunate that ActivePerl's PPM packages don't include tests. It's not because it tested OK on the platform they built the distribution on, that it'll be fine on the computer you want to work on. There could be external DLLs that are missing, or different, or there could be platform differences between for example WinXP and Win98. Take Win32::NetResource for example, there's no way it can run on Win98, because the DLL it depends on, is just a stub on Win98.