Fix problems with DexClassLoaderTest.

This fixes a problem with the DexClassLoaderTest where oat files from
previous tests were being cached and reused for later tests. Now each
test uses its own directory for loading-test.dex and friends so they
are treated as separate dex files from test to test.

As part of this fix, the helper functions for the tests were
reorganized.

Bug: 21033982
Change-Id: I66d011109259c0c3688b64b083d4ef1ed70feae2