test: check unbounded memory leak in pthread detach

In pthread_leak.detach test, compare the memory usage after two same
scenarios - all children threads have exited. Thus, test emulated by
NativeBridge, which may reserve a memory pool, will pass if it's not
unbounded leak.

Bug: https://issuetracker.google.com/37920774
Test: CtsBionicTestCases
Change-Id: Ia51aa67bacb588284747652b36ea5d8e85bea832