A developer version of Test::Aggregate is now on its way to the CPAN. This release appears to fix my biggest nit (the Test::Builder overrides were not in their own package), and some serious issues which made debugging large test suites problematic.
Hmm, set_filenames behavior was Schwern's suggestion and I forgot to include that in the docs. I'll update that for the next release.
There are some ugly globals in the Test::Builder overrides and I'll hopefully fix them in the next release. On the other hand, we are talking about a singleton. Still, it will be nice to protect them.