[scudo] Optimize scudo test string allocation

When the underlying vector becomes full, it resizes, remaps, and then copies over the old data. To avoid thes excess allocations, allow reservation from the backing vector.

Differential Revision: https://reviews.llvm.org/D135119

GitOrigin-RevId: 119f977d9e3f8af9f5ac54270e828e354452f903
2 files changed