Add tests to increase spec coverage

ci: run downstream nokogiri tests (#154)

ci: Run downstream html5gum tests (#152)

ci: Run downstream parse5 tests (#155)

update hr-in-select tests with parser errors

Support <hr>-in-<select>

Provide errors for test introduced in #163

Test <div><table><svg><foreignObject><select><table><s>

Fix typos in descriptions and errors

Implement linting for html5lib-tests

Correct errors in tests recently introduced

Remove leading whitespcae before test lines

Remove duplicate tests

Upstream new WebKit Math and namespaced attribute tests

Correct another <template> test

Upstream WebKit tests

ci: add a skeleton github actions workflow for downstream projects