Add automatic testing of responses to empty messages

[neiljp]

This is an alternative testing-based (rather than default-behavior-based) approach to #58, which also could fix #55 - though again, via tests (test_lib.py), rather than lib.py.

This follows the idea I saw introduced with test_bot_usage. I ended up overriding the function in a few bots, though I had originally planned to keep it as local to test_lib.py as possible.

@showell You seem to be taking a lead on tests atm, so tagging you for review/comments.

[zulipbot]

Hello @neiljp, it seems like you have referenced an issue in your pull request, but you have not referenced that issue in your commit message(s). When you reference an issue in a commit message, it automatically closes the corresponding issue when the commit is merged.

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commit fabd5e450374c8dde65ec35f02140383940fe146
Author: zulipbot
Date:   Sat Mar 18 13:42:40 2017 -0700

    pull requests: Check PR commits reference when issue is referenced.

    Fixes #51

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