Add seed to test of external node unit test, and indirectly have the …

…external node point to it

Previously I was getting an error about the columns for the external model
not being fetchable from the database via the macro `get_columns_in_relation`.
By creating a seed for the tests, which creates a table in postgres, we can then
tell the external model that it's database schema and identifier (the relation)
is that table from the seed without make the seed an actual dependency of the
external model in the dag.

tests/functional/unit_testing/test_unit_testing.py

@@ -266,17 +266,25 @@ def test_nonexistent_seed(self, project):
         - {user_id: 4, is_valid_email_address: false}
 """
 
+external_node_seed_csv = """user_id,email,emtail_top_level_domain
+1,"example@example.com","example.com"
+"""
+
 
 class TestUnitTestExternalNode:
     @pytest.fixture(scope="class")
-    def external_model_node(self):
+    def external_model_node(self, unique_schema):
         return ModelNodeArgs(
             name="external_model",
             package_name="external_package",
-            identifier="test_identifier",
-            schema="test_schema",
+            identifier="external_node_seed",
+            schema=unique_schema,
         )
 
+    @pytest.fixture(scope="class")
+    def seeds(self):
+        return {"external_node_seed.csv": external_node_seed_csv}
+
     @pytest.fixture(scope="class")
     def models(self):
         return {
@@ -297,5 +305,6 @@ def test_unit_test_ext_nodes(
         external_nodes.add_model(external_model_node)
         get_plugin_manager.return_value.get_nodes.return_value = external_nodes
 
+        run_dbt(["seed"], expect_pass=True)
         results = run_dbt(["test", "--select", "valid_emails"], expect_pass=True)
         assert len(results) == 1