Change test harness example to session scoped

docs/tutorials/testing.md

@@ -40,11 +40,14 @@ from prefect.utilities.testing import prefect_test_harness
 
 from my_flows import my_favorite_function
 
-@pytest.fixture(autouse=True)
+@pytest.fixture(autouse=True, scope="session")
 def prefect_test_fixture():
     with prefect_test_harness():
         yield
 
 def test_my_favorite_function(prefect_test_fixture):
     assert my_favorite_function().result() == 42
-```
+```
+
+!!! note Session scoped fixture
+    In this example, the fixture is scoped to run once for the entire test session. In most cases, you will not need a clean database for each test and just want to isolate your test runs to a test database. Creating a new test database per test creates significant overhead, so we recommend scoping the fixture to the session. If you need to isolate some tests fully, you can use the test harness again to create a fresh database.