Functional tests missing for the attachment selector macro

The attachment selector macro is currently poorly covered by functional tests.
The only test using it is org.xwiki.user.test.ui.UserProfileIT#changeAvatarImage through org.xwiki.user.test.po.ProfileUserProfilePage#changeAvatarImage

In addition, the interaction is defined directly in ProfileUserProfilePage instead of being define in a page object in xwiki-platform-attachment-test-pageobjects.