[TSan] Update test values

Summary:
This test was broken by the tail duplication logic being changed in
r311139, update the test values and add a note about how to properly run
a benchmark to verify that the values are safe to update.

Reviewers: vitalybuka

Reviewed By: vitalybuka

Subscribers: dvyukov, kubamracek

Differential Revision: https://reviews.llvm.org/D36889

llvm-svn: 311189