Add automated test for "Edit a section of a page with Text Editor"

The manual test is here: Edit a section of a page with Text Editor

Existing automated coverage

• UserProfileIT#changeDefaultEditor: Sets "Default Editor to use" to Text in the user profile preferences and asserts the saved value.
• SectionEditIT#sectionEditInWikiEditorWhenSyntax2x: Section edit links open only the section content (reached via editSection().editWiki(), i.e. from the WYSIWYG editor).
• SectionEditIT#sectionSaveDoesNotOverwriteTheWholeContentWhenSyntax20: Saving a section (unmodified) keeps the rest of the page.

Missing automated coverage

• Set the user's "Default Editor to use" preference to Text (PreferencesEditPage#setDefaultEditorText).
• On a page with several sections, click a section edit pencil and assert the wiki (Text) editor opens directly with only that section's content.
• Modify the section text and Save & View.
• Assert the modified section is displayed and the other sections are unchanged.

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/SectionEditIT.java

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