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

(cherry picked from commit cf17b487)

Change-Id: Ib47b26975489b397783e29d65f67fe775b060589