Fixed crazy make clean bug, and add if to prevent the bug from coming back.

I once again foolishly "blamed the compiler" (in this case dzil) instead of my
own code being the problem. make clean was being called, but it was me who was
calling it. I previously have only tested fetchware with
FETCHWARE_RELEASE_TESTING pretty much always being set during development,
because it provides for more thurough testing. And I was calling make_clean()
inside t/App-Fetchware-build.t, which so happend to alphabetically be the very
first test file called. These make_clean calls should only happen inside a
subtest where skip_all_unless_release_testing() has been called to skip the
subtest when *not* FETCHWARE_RELEASE_TESTING. So, I moved those calls inside the
next subtest, so skip_all_unless_release_testing() will be called.

I also added some if statements to make_clean() so that if its called inside of
Fetchware's own directory it will BAIL_OUT with an appropriate error message.