[scudo][standalone] Unmap memory in tests

Summary:
The more tests are added, the more we are limited by the size of the
address space on 32-bit. Implement `unmapTestOnly` all around (like it
is in sanitzer_common) to be able to free up some memory.
This is not intended to be a proper "destructor" for an allocator, but
allows us to not fail due to having no memory left.

Reviewers: morehouse, vitalybuka, eugenis, hctim

Reviewed By: morehouse

Subscribers: delcypher, jfb, #sanitizers, llvm-commits

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D63146

llvm-svn: 363095