[SanitizerCommon] Test `CombinedAllocator::ForEachChunk()` in unit tests.

Summary:

Previously we weren't testing this function in the unit tests.

Reviewers: kcc, cryptoad, dvyukov, eugenis, kubamracek

Subscribers: #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D54861

llvm-svn: 348260