Added a unit-test for the system check.

setup.py

@@ -25,7 +25,7 @@ setup(
     install_requires=["twisted[tls]>=22.4", "autobahn>=22.4.2", "asgiref>=3.5.2,<4"],
     python_requires=">=3.7",
     setup_requires=["pytest-runner"],
-    extras_require={"tests": ["hypothesis", "pytest", "pytest-asyncio"]},
+    extras_require={"tests": ["hypothesis", "pytest", "pytest-asyncio", "django"]},
     entry_points={
         "console_scripts": ["daphne = daphne.cli:CommandLineInterface.entrypoint"]
     },

tests/test_checks.py

@@ -0,0 +1,21 @@
+import django
+from django.conf import settings
+from django.test.utils import override_settings
+
+from daphne.checks import check_daphne_installed
+
+
+def test_check_daphne_installed():
+    """
+    Test check error is raised if daphne is not listed before staticfiles, and vice versa.
+    """
+    settings.configure(
+        INSTALLED_APPS=["daphne.apps.DaphneConfig", "django.contrib.staticfiles"]
+    )
+    django.setup()
+    errors = check_daphne_installed(None)
+    assert len(errors) == 0
+    with override_settings(INSTALLED_APPS=["django.contrib.staticfiles", "daphne"]):
+        errors = check_daphne_installed(None)
+        assert len(errors) == 1
+        assert errors[0].id == "daphne.E001"