Fix pthread_leak test flakiness.

Stop allocating and deallocating memory as part of the test itself.

There's still the fopen, but since our stdio reuses existing structs,
that doesn't seem to be a problem in practice.

Bug: http://b/67077411
Test: ran tests with --gtest_repeat=1000
Change-Id: I99de5de0911161ec04afe75653075f1ccefb01a5