[6.0] Add swift-testing to update-checkout-config.json

To release/6.0 branch.

Author: rintaro

utils/update_checkout/update-checkout-config.json

@@ -182,6 +182,7 @@
                 "swift-syntax": "release/6.0",
                 "swift-system": "1.3.0",
                 "swift-stress-tester": "release/6.0",
+                "swift-testing": "main",
                 "swift-corelibs-xctest": "release/6.0",
                 "swift-corelibs-foundation": "release/6.0",
                 "swift-foundation": "release/6.0",