Include build dependencies in Perl lib path for tests

This is kinda ugly IMO as the path will get inserted twice due to inheritance or something. I'm not 100% if Module::Build has the same issue as I couldn't get any tests to show one way or the other; I literally couldn't find a module using Build.PL which had test_requires that weren't already installed for me. If it does have the issue, a similar fix is needed.

I also noticed in testing that MakeMaker will run instead of MB in the event of a distribution having both (f.e. Carton). As MB is newer, it probably makes more sense to put that first. Either way, this pull request should solve #490 and I'm going to leave the branch open to see what I can do about #442 and the potential issue in Module::Build.