Carry package dbs into testsuite

Prior to this patch, the testsuite used your global package db, and
sometimes, the inplace package db, which led to errors if you didn't
have old-time installed globally. They looked like this:

BuildDeps/SameDepsAllRound:          Cabal result was Result {successful = False, success = Failure, outputText = "\"/home/enolan/cabal/Cabal/tests/Setup configure --user -w  /home/enolan/.nix-profile/bin/ghc\" in PackageTests/BuildDeps/SameDepsAllRound\nConfiguring SameDepsAllRound-0.1...\nSetup: At least the following dependencies are missing:\nold-time -any\n"}