Add automated test for "Create a Terminal page"

The manual test is here: Create a Terminal page

Existing automated coverage

• CreatePageAndSpaceIT#createSpaceAndPage: Test 3: from an existing (terminal) page, Create, fill the name, check Terminal, create, Save & View; page name asserted. The target space is passed explicitly, so the default location is not checked.
• CreatePageAndSpaceIT#testCreateUIInteraction: Terminal checkbox interaction and parent validation in the create form.

Missing automated coverage

• As an advanced user, create a terminal page "a" from the Create UI (Terminal checked) and save it
• Assert "a" is terminal (reference Space.a, not Space.a.WebHome)
• On page "a" click Create, assert the proposed parent location is a's parent space (not "a"), enter title "b", create and save
• Assert "b" is created as a sibling of "a" (same parent space) and that "a" has no child pages

Suggested location: xwiki-platform-core/xwiki-platform-flamingo/xwiki-platform-flamingo-skin/xwiki-platform-flamingo-skin-test/xwiki-platform-flamingo-skin-test-docker/src/test/it/org/xwiki/flamingo/test/docker/CreatePageAndSpaceIT.java

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