adds single snippet cfg for testing:

Author: chalmerlowe

.kokoro/snippets-3.13.cfg

@@ -0,0 +1,7 @@
+# Format: //devtools/kokoro/config/proto/build.proto
+
+# Only run this nox session.
+env_vars: {
+    key: "NOX_SESSION"
+    value: "snippets-3.13"
+}