Ensure user name is language neutral in RDFa and update tests to parse RDFa

Problem/Motivation

rdf_preprocess_username() makes sure the name of a user is output as a string without language:

  // Because lang is set on the HTML element that wraps the page, the
  // username inherits this language attribute. However, since the username
  // might not be transliterated to the same language that the content is in,
  // we do not want it to inherit the language attribute, so we set the
  // attribute to an empty string.

This function isn't used on user profile page, instead rdf_preprocess_user() is used, but it doesn't include this language stripping.

Proposed resolution

Add same functionality of rdf_preprocess_username() in rdf_preprocess_user() so the RDF outputs match.

This issue will also include tests for user output, split from #1869914: Refactor RdfMappingDefinitionTestCase and split it in dedicated test cases.