[PATCH v2] test: Fix HTML rendering test

[Ethan Glasser-Camp]

Austin Clements <amdragon at MIT.EDU> writes:

> 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.

This fixes the failing test on my machine.

Ethan