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
why would you do this? now you don't even know what you are verifying.
and we are dealing with EB installing python packages with an EB installed python and setuptools, right?