verify installation of a dummy Python package before actually installing a Python package

After spending about 2h on trying to figure out why a particular `setuptools` installation was blatently ignoring `--prefix`, I was sufficiently frustrated to come up with this: a verification of whether whichever `setuptools` is active honors `--prefix` before actually installing a Python package with it.

Up until now, I've run into two different ways of making `setuptools` blatently ignore `--prefix`:

 * via the `distutils.cfg` or `$HOME/.pydistutils.cfg` configuration files which effectively override any specified `--prefix` comand line option (cfr. https://docs.python.org/2/install/#distutils-configuration-files)
 * by using a `setuptools` installation that was installed with `--user` into `$HOME/.local`, causing it to install everything to `$HOME/.local` regardless of what `--prefix` specifies (yes, you read that right)

Since there's little we can do to prevent the use of a (basically broken) `setuptools` installation (there's no way I'm aware of in which we can install our own and be 100% sure it'll be used), we better have a way of verifying whether the `active` setuptools does the right thing.

related: https://github.com/hpcugent/easybuild/issues/31, https://github.com/hpcugent/easybuild-easyblocks/issues/373 and last nights discussion with @kcgthb on the `#easybuild` IRC channel

@stdweird, @JensTimmerman: thoughts on this?

cc: @kcgthb, @chwilk, @DimitriSteyaert, @pforai, @ocaisa
define outside loop?