tests/test-gendoc-sv.t

templater: teach template loader to use open_template() function

The template loader can apparently load templates from relative paths,
so I needed to add that support to `open_template()`.

This takes the number of failing tests with PyOxidizer from 54 to 34.

Differential Revision: https://phab.mercurial-scm.org/D8907

#require docutils gettext

  $ $TESTDIR/check-gendoc sv
  checking for parse errors