test: add explicit credentials and project to test (googleapis#1372)

* test: add explicit credentials and project to test

Tests need to be able to run in environments without any default credentials.
This requires the test to explictly set the credentials and the project that
should be used.

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

---------

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>

Author: olavloite

.kokoro/presubmit/presubmit.cfg

@@ -1,7 +1,7 @@
 # Format: //devtools/kokoro/config/proto/build.proto
 
-# Only run a subset of all nox sessions
+# Disable system tests.
 env_vars: {
-    key: "NOX_SESSION"
-    value: "unit-3.8 unit-3.12 cover docs docfx"
+    key: "RUN_SYSTEM_TESTS"
+    value: "false"
 }

tests/unit/spanner_dbapi/test_connection.py

@@ -830,7 +830,14 @@ def test_custom_database_role(self):
         from google.cloud.spanner_dbapi import connect
 
         role = "some_role"
-        connection = connect("test-instance", "test-database", database_role=role)
+        connection = connect(
+            "test-instance",
+            "test-database",
+            project="test-project",
+            database_role=role,
+            credentials=AnonymousCredentials(),
+            client_options={"api_endpoint": "none"},
+        )
         self.assertEqual(connection.database.database_role, role)
 
     def test_invalid_custom_client_connection(self):