Sorry, I don’t want to monkey-patch find_executable during tests.  We want the tests to work with the real code, with the same behavior and errors as real setup.py calls.  If a test does not work with python -E, then it should be skipped.

(Implementation idea:
  skip_if_empty_env = unittest.skipUnless(
      os.environ, 'test cannot run with an empty environment')
using sys.flags is not possible due to backward compatibility policy)