[scudo] Deallocate the AllocatorRingBuffer too in unmapTestOnly

The AllocatorRingBuffer is allocated dynamically when Allocator is
initialized. This patch adds a corresponding deinitialization call in
unmapTestOnly, to avoid running out of virtual memory if the tests are run
a large number of times on memory-constrained platforms.

Reviewed By: Chia-hungDuan

Differential Revision: https://reviews.llvm.org/D149266