[scudo] Add DoubleFreeFromSecondary death test (#242398)
Complement DoubleFreeFromPrimary with a death test verifying that
double-freeing a cached secondary allocation fails with "invalid chunk
state" (or faults under MTE, which marks cached secondary entries
MAP_NOACCESS). Guard the test on configurations with an active secondary
cache, since uncached blocks are unmapped on the first deallocate() and
fault when loading the chunk header instead.
Also remove an unused condition_variable.h include.
Used Gemini to review my change.
