Add debugging for b/62653020

We are seeing errors reading out/soong/build.ninja when running
ART tests.  Add a hack to copy out/soong/build.ninja to DIST_DIR
on failure so we can see the contents.

Test: Manually corrupt build.ninja, run testrunner.py with and without
      DIST_DIR set.
Bug: 62653020

Change-Id: I6d83b87d56b6787a98fec8b244f224d44cb85bc1