Cleanup some django.contrib.gis tests and fix random failure.

I wanted to fix the random `test_make_line` failure happening on some builds — see http://djangoci.com/job/django-pull-requests/1080/database=postgis,python=python3.4/testReport/junit/django.contrib.gis.tests.geoapp.tests/GeoQuerySetTest/test_make_line/ for instance — and I realized that the assertion functions used needed some cleanup too. I have put it all in this PR.

The `tolerance=10e-5` parameter might look arbitrary, but since it's the minimum precision of the ref_line coordinates I think there is no need to check with a better precision.