|P is for Practical|
Your older vs newer tags are wrong — there are at least three well used installers — and you surely use them to attempt convey false impressions of unfitness. Don't do that.
Nonsense. MM doesn't care one bit. It has no expectations at all in this area.
(Upd: By default, it would install .pm found in the room directory. One could say it expects you not to put test libraries there, the opposite of what you said. )
Just like you would have to do with any other script that wants to use uninstalled modules, the script needs to tell Perl where to find the modules.
Your syntax is wrong, though. It makes assumption about the current directory. Running the test from the directory in which its located would fail.
That's completely false. I imagine that 100% of testers try to use Makefile.PL, not 3%. They just might not try it first.
(Upd: Furthermore, that shouldn't be taken to mean Build.PL is preferred. It makes more sense to try Build.PL first because users of Build.PL often provide a Makefile.PL for compatibility. )