chore(tests): update kokoro continuous test config (#303)

The continuous kokoro test should use the [same configuration as the
pre-submit](https://github.com/googleapis/python-test-utils/blob/main/.kokoro/presubmit/presubmit.cfg)

Author: daniel-sanche

packages/google-cloud-testutils/.kokoro/continuous/continuous.cfg

@@ -1 +1,6 @@
-# Format: //devtools/kokoro/config/proto/build.proto
+# Format: //devtools/kokoro/config/proto/build.proto
+
+env_vars: {
+  key: "NOX_SESSION"
+  value: "blacken mypy check_lower_bounds"
+}