Add run step to function test of unit test with external nodes

This is necessary because when executing a unit tests, the columns
associated with a model in the database are retrieved. For this to
be possible, the model must exist in the database, thus we must
run the associated models at least once first.

tests/functional/unit_testing/test_unit_testing.py

@@ -326,5 +326,7 @@ def test_unit_test_ext_nodes(
 
         # `seed` need so a table exists for `external_model` to point to
         run_dbt(["seed"], expect_pass=True)
+        # `run` needed to ensure `top_level_domains` exists in database for column getting step
+        run_dbt(["run"], expect_pass=True)
         results = run_dbt(["test", "--select", "valid_emails"], expect_pass=True)
         assert len(results) == 1