test: add simple test for default 404 response

tests/test_default_404.py

@@ -0,0 +1,11 @@
+from fastapi import FastAPI
+from fastapi.testclient import TestClient
+
+
+def test_default_404():
+    app = FastAPI()
+    client = TestClient(app)
+
+    response = client.get("/non-existing-route")
+    assert response.status_code == 404
+    assert response.json()["detail"] == "Not Found"