test/test.py: Update clean_result() to normalize user agents

Catch up with the USER_AGENT changes from 3f9adb5 (feed: Add the
digest setting for multi-entry email, 2013-04-13).

Signed-off-by: W. Trevor King <wking@tremily.us>
test/gmane/2.expected: Remove some whitespace

I'm not sure which version of html2text I used to generate the initial
expected results, but this version was generated with html2text
3.200.3.

Signed-off-by: W. Trevor King <wking@tremily.us>
test/gmane/3: Add tests for HTML generation

Signed-off-by: W. Trevor King <wking@tremily.us>