Adds tests for the addPortletLinkHandler, which should
add an icon element to items added via 'mw.util.addPortletLink'
and conditionally move elements from the toolbar into
the more menu at narrow widths.
- Adds tests for dropdownMenu.js
- Adds associated Jest snapshots
- Stubs the mediawiki.page.ready module
- Updates mockMediawiki.js to 4.6.0
- Adds global mock for mw.util.showPortlet()
- Cleans up tests using local versions of these mocks
Bug: T314798
Change-Id: I81394d840321916756a41a23c40d96c4b6341931
