Fix test_distutils to not fail when running 'make test' from a Python build

Fix test_distutils to not fail when running 'make test' from a Python build
directory that is not the source directory (ie, one created using
'/path/to/source/configure'.) Leaves this test very slightly degraded in
that particular case, compared to the build-in-sourcedir case, but that case
isn't a particularly strong test either: neither test the actual path that
will be used after installing. There isn't a particularly good way to test
this, and a poor test beats a failing test.