Add unit test to enforce unique file names.

This patch adds a unit test to ensure we have uniquely named
files across each of the element/target directories. Because
DIB copies these files into a common namespace files can
by cryptically overwritten thus causing a variety of failures.

Simply checking for uniquely named files when running unit
tests should help eliminate future issues here.

Change-Id: Ic22f9eb5e002a0cfe94942e7210eb73231560e82
Partial-bug: #1321849
Merge "Add unit test to enforce unique file names."