Update tests for missing migrations

The test passed originally ran with the suite, and managed to fail
independently. This updates the test itself to make use of Django's new
migration option `--check` and to add a separate step to the build
command.

gh-145

Makefile

@@ -64,6 +64,7 @@ test-all:
 check: clean-build clean-pyc clean-test lint test-coverage
 
 build: clean  ## Create distribution files for release
+	pytest -k test_no_missing_migrations
 	python setup.py sdist bdist_wheel
 
 release: build  ## Create distribution files and publish to PyPI

tests/test_migrations.py

@@ -2,30 +2,13 @@
 Tests that migrations are not missing
 """
 
-try:
-    from io import StringIO
-except ImportError:
-    from StringIO import StringIO
-
 import pytest
 
 from django.core.management import call_command
-from django.conf import settings
+
 
 @pytest.mark.django_db
 def test_no_missing_migrations():
     """Check no model changes have been made since the last `./manage.py makemigrations`.
-
-    Pulled from mozilla/treeherder #dd53914, subject to MPL
     """
-    try:
-        with pytest.raises(SystemExit) as e:
-            # Replace with `check_changes=True` once we're using a Django version that includes:
-            # https://code.djangoproject.com/ticket/25604
-            # https://github.com/django/django/pull/5453
-            call_command('makemigrations', interactive=False, dry_run=True, exit_code=True)
-        assert str(e.value) == '1'
-    except TypeError:
-        out = StringIO()
-        call_command('makemigrations', interactive=False, dry_run=True, check=True, stdout=out)
-        assert str(out.getvalue()) == 'No changes detected\n'
+    call_command('makemigrations', check=True, dry_run=True)