chg: refactor remaining translation tests to use XPathHelper, add docs
chg: refactor remaining translation tests to use XPathHelper, add docs

- Update TestTranslations and TestTranslationsSurvey that were using
  string constants to use XPathHelper so that the assertions being made
  are easier to follow.
- Add XPathHelper method docstrings to describe what they assert.

XLSForm/pyxform,XLSForm/pyxform