[NFC][LSAN] Limit the number of concurrent threads is the test

Test still fails with D88184 reverted.

The test was flaky on https://bugs.chromium.org/p/chromium/issues/detail?id=1206745 and
https://lab.llvm.org/buildbot/#/builders/sanitizer-x86_64-linux

Reviewed By: morehouse

Differential Revision: https://reviews.llvm.org/D102218