bug#20994: python tests hang

[Glenn Morris]

Package: emacs
Version: 25.0.50

Following yesterday's python changes, "make -C test/automated python-tests"
hangs forever for me with
    
    passed  178/208  python-shell-make-comint-2
    Can't guess python-indent-offset, using defaults: 4
    Warning (python): Python shell prompts cannot be detected.
    If your emacs session hangs when starting python shells
    recover with `keyboard-quit' and then try fixing the
    interactive flag for your interpreter by adjusting the
    `python-shell-interpreter-interactive-arg' or add regexps
    matching shell prompts in the directory-local friendly vars:
      + `python-shell-prompt-regexp'
      + `python-shell-prompt-block-regexp'
      + `python-shell-prompt-output-regexp'
    Or alternatively in:
      + `python-shell-prompt-input-regexps'
      + `python-shell-prompt-output-regexps'

I have to use ctrl-C to kill it.
A test should not hang forever in that way, it should just fail.

(M-x run-python works fine interactively.)