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_correct/admin.py

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+from django.contrib import admin
+
+# Register your models here.

tests/test_project/app_correct/apps.py

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+from django.apps import AppConfig
+
+
+class AppCorrectConfig(AppConfig):
+    name = "app_correct"

tests/test_project/app_correct/migrations/0001_initial.py

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+# Generated by Django 2.1.4 on 2019-03-19 21:08
+
+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_correct/migrations/0002_foo.py

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+# Generated by Django 2.1.4 on 2019-03-19 21:08
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [("app_correct", "0001_initial")]
+
+    operations = [
+        migrations.AddField(
+            model_name="a", name="new_null_field", field=models.IntegerField(null=True)
+        )
+    ]

tests/test_project/app_correct/models.py

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+from django.db import models
+
+
+class A(models.Model):
+    null_field = models.IntegerField(null=True)
+    new_null_field = models.IntegerField(null=True)

tests/test_project/app_correct/tests.py

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+from django.test import TestCase
+
+# Create your tests here.

tests/test_project/app_correct/views.py

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+from django.shortcuts import render
+
+# Create your views here.