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/

tests/python/pants_test/backend/python/test_test_builder.py

@@ -29,8 +29,11 @@ def _cache_current_interpreter(self):
     # We only need to cache the current interpreter, avoid caching for every interpreter on the
     # PATH.
     current_interpreter = PythonInterpreter.get()
+    current_id = (current_interpreter.binary, current_interpreter.identity)
     for cached_interpreter in cache.setup(filters=[current_interpreter.identity.requirement]):
-      if cached_interpreter == current_interpreter:
+      # TODO(John Sirois): Revert to directly comparing interpreters when
+      # https://github.com/pantsbuild/pex/pull/31 is in, released and consumed by pants.
+      if (cached_interpreter.binary, cached_interpreter.identity) == current_id:
         return cached_interpreter
     raise RuntimeError('Could not find suitable interpreter to run tests.')