[scudo] Show the resident pages from the secondary cache. (#170568)

Move the test utility to get the resident pages into linux.cpp.

Add some specific tests for this function.

Add displaying of resident pages from in the secondary cache when
calling getStats.

NOKEYCHECK=True
GitOrigin-RevId: efd472c88d7bed50615eab6db844a766d8f1964f
6 files changed