Exclude things in python/scripts from coverage reporting for now.

I can't get the coveragereport.py --path-alias invocation to work
correctly, but I expected it would be something like:

  share/dnanexus/lib/python2.7/sitepackages/dxpy*.egg/EGG-INFO/scripts => src/python/scripts
Protect coverage reporting from os.chdir.

Also factor out some shared methods that were used during the tests.