BibKnowledge: lxml port get_kbt_items_for_bibedit

* Replaces usage of `libxml2` and `libxslt` with `lxml` in
  `get_kbt_items_for_bibedit` function.

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
BibKnowledge: regression tests fix

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
testutils: regression tests fix

* Addresses issue with broken 'xm' output format.

* Improves landing page compatibility with regression tests.

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>