Ignore leading and trailing whitespace in Emmet content check

[BTWS2]

• title has in this example a leading whitespace and fails because of it. The Emmet check should have the same behavior as the regular check: https://github.com/BTWS2/judge-html/blob/main/docs/pages/element-class.md#has_content. @QuintenVervynck mentioned in Check content of indexed tag with emmet #156 (comment) the library has limitations.
  https://naos.ugent.be/en/submissions/982306/
  https://github.ugent.be/brewille/oef_websites/tree/master/html-exercises/lorem_ipsum

html.make_item_from_emmet('The <title> tag has right content',
                          "head>title{A panagram}")

• https://naos.ugent.be/en/submissions/982334/

• https://naos.ugent.be/nl/submissions/982641/
  https://github.ugent.be/brewille/oef_websites/blob/master/html-exercises/create_table6/evaluation/evaluator.py

• Tests were added