Add tests for the makemigrations override

tests/test_project/makemigrations_backward_incompatible_migration_missing/apps.py

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+from django.apps import AppConfig
+
+
+class MakemigrationsBackwardIncompatibleMigrationMissingConfig(AppConfig):
+    name = "tests.test_project.makemigrations_backward_incompatible_migration_missing"

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

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+# Generated by Django 2.2 on 2020-06-26 17:49
+
+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",
+                    ),
+                ),
+                ("x", models.IntegerField()),
+            ],
+        ),
+    ]

tests/test_project/makemigrations_backward_incompatible_migration_missing/models.py

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+from django.db import models
+
+
+class A(models.Model):
+    x = models.IntegerField()
+    new_field = models.CharField(null=False, default="empty", max_length=255)

tests/test_project/makemigrations_correct_migration_missing/apps.py

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+from django.apps import AppConfig
+
+
+class MakemigrationsCorrectMigrationMissingConfig(AppConfig):
+    name = "tests.test_project.makemigrations_correct_migration_missing"

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

@@ -0,0 +1,28 @@
+# Generated by Django 2.2 on 2020-06-26 15:57
+
+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",
+                    ),
+                ),
+                ("x", models.IntegerField()),
+            ],
+        ),
+    ]

tests/test_project/makemigrations_correct_migration_missing/models.py

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+from django.db import models
+
+
+class A(models.Model):
+    x = models.IntegerField()
+    new_field = models.CharField(null=True, max_length=255)