Add automated test for "Export Import a document with defined permissions"

The manual test is here: Export Import a document with defined permissions

Existing automated coverage

• XARExportIT#exportXARAll: More actions > Export > Export as XAR of nested pages (export modal only, content of the XAR not checked).
• XARImportIT#testImportWithHistoryWhenNoPage: Administer Wiki > Content > Import: upload and import a prebuilt XAR, main wiki only.
• UsersGroupsRightsManagementIT#denyUserRightAtPageLevelWhenAllowedToGroupAtPageLevel: Setting rights from Administer Page > Users & Rights > Rights: Page and checking their effect as a non-admin user (main wiki and subwiki).

Missing automated coverage

• Create nested page 1/2/3/4 (no intermediate pages), restrict View to XWikiAdminGroup from Administer Page > Rights: Page
• Create a global user and assert that user cannot view page 4
• As Admin, export page 4 with More actions > Export > Export as XAR
• Create a subwiki that allows global users and import the XAR from its Administer Wiki > Content > Import
• Assert the imported page carries the XWikiRights object, is viewable by Admin and not viewable by the global user

Suggested location: xwiki-platform-core/xwiki-platform-administration/xwiki-platform-administration-test/xwiki-platform-administration-test-docker/src/test/it/org/xwiki/administration/test/ui/XARImportIT.java

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