[scudo] Use anonymous namespace for test helper code (#157207) Tests can be at top-level or inside an anonymous namespace, doesn't matter. But putting their helper code inside anonymous namespaces both makes the code compatible with compiling using -Wmissing-declarations and might let the compiler optimize the test good a bit better. GitOrigin-RevId: 8ddd8045ac3e74ef3273e50973534b35c0620d52