pager: add tests
We've never had tests for pager, and I want to start experimenting
with ways to clean up the implementation and make it a bit more
graceful.
{header}
<title>Error</title>
<description>Error</description>
<item>
<title>Error</title>
<description>{error|escape}</description>
<guid>https://mercurial-scm.org/#error</guid>
</item>
</channel>
</rss>