Fixup test_test_builder.py interpreter caching.

Comparing PythonInterpeters directly is currently broken for systems
that have atime turned on for the filesystem hosting the python binary.
This localizes the 'correct' equality check proposed in:
  https://rbcommons.com/s/twitter/r/1477/

Testing Done:
The same change was tested against OSX here: https://travis-ci.org/pantsbuild/pants-for-travis-osx-ci/builds/43799766

This change went green against linux CI here: https://travis-ci.org/pantsbuild/pants/builds/43803116

Bugs closed: 865

Reviewed at https://rbcommons.com/s/twitter/r/1478/