Add automated test for "Homepage Tour"

The manual test is here: Homepage Tour

Existing automated coverage

• TourApplicationIT#verifyTourFeatures: Generic tour mechanics on a custom tour: Next/Previous, close (the X button), resume at the last seen step, End tour, resume after end

Missing automated coverage

• With the flavor tour XAR installed and fresh browser storage, open the Home page and assert the tour starts with Welcome to XWiki
• Click Next through Breadcrumbs, Page Menu, Header, Panels, Page Content, Page Tabs to Tour Complete, asserting Prev works at each step
• Close with x, click Show tour and assert the last seen step is displayed
• Assert the documentation link of the last step targets the XWiki Features user guide
• Click End tour and assert the tour closes and the restart button is shown

Suggested location: xwiki-platform-core/xwiki-platform-tour/xwiki-platform-tour-test/xwiki-platform-tour-test-docker (new HomepageTourIT, with a dependency on xwiki-platform-distribution-flavor-tour)

Found by comparing the XWiki 18.4.4 manual test runs against the automated tests on master (a0055f10a1a).