Tests: add test project

* manage.py file at the root of the project
* targetting the project located at tests/test_project/settings.py

That allows testing the linter through just calling it on the project itself.
Using the new method coming soon: `python manage.py lintmigrations`

tests/test_project/app_add_not_null_column/migrations/0001_create_table.py

@@ -0,0 +1,28 @@
+# Generated by Django 2.1.4 on 2019-03-19 20:59
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    initial = True
+
+    dependencies = []
+
+    operations = [
+        migrations.CreateModel(
+            name="A",
+            fields=[
+                (
+                    "id",
+                    models.AutoField(
+                        auto_created=True,
+                        primary_key=True,
+                        serialize=False,
+                        verbose_name="ID",
+                    ),
+                ),
+                ("null_field", models.IntegerField(null=True)),
+            ],
+        )
+    ]

tests/test_project/app_add_not_null_column/migrations/0002_add_new_not_null_field.py

@@ -0,0 +1,17 @@
+# Generated by Django 2.1.4 on 2019-03-19 21:05
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [("app_add_not_null_column", "0001_create_table")]
+
+    operations = [
+        migrations.AddField(
+            model_name="a",
+            name="new_not_null_field",
+            field=models.IntegerField(default=1),
+            preserve_default=False,
+        )
+    ]