Add funtional tests for lintmigrations command

tests/functional/test_lintmigrations_command.py

@@ -0,0 +1,31 @@
+from unittest.mock import patch
+
+from django.core.management import call_command
+from django.test import TransactionTestCase
+
+
+class LintMigrationsCommandTestCase(TransactionTestCase):
+    databases = {"default", "sqlite"}
+
+    def test_plain(self):
+        with self.assertRaises(SystemExit):
+            call_command("lintmigrations")
+
+    def test_config_file_app_label(self):
+        with patch(
+            "django_migration_linter.management.commands.lintmigrations.Command.read_config_file"
+        ) as config_fn:
+            config_fn.return_value = {"app_label": "app_correct"}
+            call_command("lintmigrations")
+
+    def test_command_line_app_label(self):
+        call_command("lintmigrations", app_label="app_correct")
+
+    def test_command_line_and_config_file_app_label(self):
+        with patch(
+            "django_migration_linter.management.commands.lintmigrations.Command.read_config_file"
+        ) as config_fn:
+            config_fn.return_value = {"app_label": "app_correct"}
+
+            with self.assertRaises(SystemExit):
+                call_command("lintmigrations", app_label="app_drop_table")