[gwp_asan] Use anonymous namespace for test helper code (#157205)

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: 1f39435567b35a6797d92cd9a24e503e6b0302fd