[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
Change-Id: I33e6d3008cd19c5b0410703b7319e5b12d14fe8e
2 files changed