test: Fix HTML rendering test

The test designed to exercise Emacs' rendering of HTML emails
containing images inadvertently assumed w3m was available under Emacs
23.  The real point of this test was to check that Emacs 24's shr
renderer didn't crash when given img tags, so use shr if it's
available, html2text otherwise (which is built in), and do only a
simple sanity check of the result.