[libc] Use anonymous namespace for test helper code (#157203)

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: 9789abb2b9ad0a3d3a2af3cc63d027ec20156650