[update-checkout] Add swift-testing to release/6.0.0 scheme

Add swift-testing:release/6.0.0

Author: rintaro

utils/update_checkout/update-checkout-config.json

@@ -234,6 +234,7 @@
                 "swift-syntax": "release/6.0.0",
                 "swift-system": "1.3.0",
                 "swift-stress-tester": "release/6.0.0",
+                "swift-testing": "release/6.0.0",
                 "swift-corelibs-xctest": "release/6.0.0",
                 "swift-corelibs-foundation": "release/6.0.0",
                 "swift-foundation": "release/6.0.0",