Store failure thumbnails in CWD/out/failures

When a test fails in layoutlib, store the resulting png files in
out/failures subdirectory of the current working directory.
That avoids the risk of collisions if tests for several branches
of layoutlib are run at the same time.

Test: Run tests in layoutlib with at least one failing test
Change-Id: I31594a871e481e6aa410a165926ce544dc7ddcf2
(cherry picked from commit 82ebb9058479de56860f348ab969160e0d8294b6)