Made migrations tests write to a temporary directory.

This is preferrable to writing in the current working directory because
it eliminates the risk to leak unwanted files, which can result in very
weird test failures.

Also this will help if we ever try to run these tests concurrently.

tests/migrations/test_base.py

@@ -1,8 +1,14 @@
 import os
+import shutil
+import tempfile
+from contextlib import contextmanager
+from importlib import import_module
 
+from django.apps import apps
 from django.db import connection
 from django.test import TransactionTestCase
-from django.utils._os import upath
+from django.test.utils import extend_sys_path
+from django.utils.module_loading import module_dir
 
 
 class MigrationTestBase(TransactionTestCase):
@@ -11,7 +17,6 @@ class MigrationTestBase(TransactionTestCase):
     """
 
     available_apps = ["migrations"]
-    test_dir = os.path.abspath(os.path.dirname(upath(__file__)))
 
     def get_table_description(self, table):
         with connection.cursor() as cursor:
@@ -64,3 +69,46 @@ class MigrationTestBase(TransactionTestCase):
 
     def assertFKNotExists(self, table, columns, to, value=True):
         return self.assertFKExists(table, columns, to, False)
+
+    @contextmanager
+    def temporary_migration_module(self, app_label='migrations', module=None):
+        """
+        Allows testing management commands in a temporary migrations module.
+
+        Wrap all invocations to makemigrations and squashmigrations with this
+        context manager in order to avoid creating migration files in your
+        source tree inadvertently.
+
+        Takes the application label that will be passed to makemigrations or
+        squashmigrations and the Python path to a migrations module.
+
+        The migrations module is used as a template for creating the temporary
+        migrations module. If it isn't provided, the application's migrations
+        module is used, if it exists.
+
+        Returns the filesystem path to the temporary migrations module.
+        """
+        temp_dir = tempfile.mkdtemp()
+        try:
+            target_dir = tempfile.mkdtemp(dir=temp_dir)
+            with open(os.path.join(target_dir, '__init__.py'), 'w'):
+                pass
+            target_migrations_dir = os.path.join(target_dir, 'migrations')
+
+            if module is None:
+                module = apps.get_app_config(app_label).name + '.migrations'
+
+            try:
+                source_migrations_dir = module_dir(import_module(module))
+            except (ImportError, ValueError):
+                pass
+            else:
+                shutil.copytree(source_migrations_dir, target_migrations_dir)
+
+            with extend_sys_path(temp_dir):
+                new_module = os.path.basename(target_dir) + '.migrations'
+                with self.settings(MIGRATION_MODULES={app_label: new_module}):
+                    yield target_migrations_dir
+
+        finally:
+            shutil.rmtree(temp_dir)