Add tests for Tag::appendContent, Tag::prependContent, Tag::clearContent
Add tests for Tag::appendContent, Tag::prependContent, Tag::clearContent

Change-Id: Ie46b1baf09ea251bd6071d605785edfe23ebde19

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui