Once part of PR #144648, follow the reviewer's advice and split into
this separate PR.
`unmap` works at page granularity, but supports an arbitrary non-zero
size as an argument, which results in possible shadow undercleaning in
the existing TSan implementation when `size % kShadowCell != 0`.
This change introduces two test cases to verify the shadow cleaning
effect in `unmap`.
- java_heap_init2.cpp: Imitating java_heap_init cpp, verify the
incomplete cleaning of meta
- munmap_clear_shadow.c: verify the incomplete cleaning of shadow
