Fix arena_test test break using glibc-2.17
Fix arena_test test break using glibc-2.17

Summary: arena_test is failing with glibc-2.17. Make it more robust

Test Plan: Run arena_test using both of glibc-2.17 and 2.2 and make sure both passes.

Reviewers: yhchiang, rven, IslamAbdelRahman, kradhakrishnan, igor

Reviewed By: igor

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D45879

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