Improve ThreadedQuarantineTest heap memory measurements.

Summary:
Warm up ASAN caches in ThreadedQuarantineTest to get more predictable
incremental heap memory usage measurements.

Reviewers: eugenis

Patch by Alex Shlyapnikov.

Subscribers: aemerson, kubabrecka, llvm-commits

Differential Revision: https://reviews.llvm.org/D28061

llvm-svn: 290371