Use blib/script/ when run under Test::Harness

[grr]

Both ExtUtils::MakeMaker and Module::Build will stage scripts that are to be installed into blib/script. The former can be specified with EXE_FILES and the latter by script_files. When make test or Build test or prove -blib is run, the staged script should instead be tested. Testing for $ENV{HARNESS_ACTIVE} can be used to tell which script location to choose.

[plicease]

I agree that testing blib/script is probably the right thing to do when run with blib enabled, and this would be a good thing do implement as an opt-in, but doing so by default will probably break existing dists that are expecting the current behavior.