Move django-admin check command to test suite

Instead of running it as part of tox, this ensures it runs as an
explicit test in the integration test suite.

tests/integration/django_test.py

@@ -0,0 +1,8 @@
+from django.core.management import call_command
+
+
+def test_django_manage_check():
+    """Runs Django's `check` management command to verify the Django project is
+    correctly configured.
+    """
+    call_command('check')

tox.ini

@@ -73,7 +73,6 @@ commands_pre =
 commands =
          unit: pytest -o junit_suite_name="{envname} unit tests" --cov-config {toxinidir}/tests/.coveragerc --cov={toxinidir}/python --cov-report=xml:reports/{envname}/coverage.xml --junitxml=reports/{envname}/unit-results.xml --verbose {posargs:tests/unittests}
 
-         integration: python -m nav.django.manage check
          {integration,functional}: nav config install {envdir}/etc
          {integration,functional}: mkdir -p {envdir}/uploads
          {integration,functional}: sed -i 's,^\#\?UPLOAD_DIR.*.,UPLOAD_DIR={envdir}/uploads,' {envdir}/etc/nav.conf