Add automated test for "Switch to source while editing in place"

The manual test is here: Switch to source while editing in place

Existing automated coverage

• InplaceEditIT#selectionRestoreOnSwitchToSource: In-place edit: paragraph selections preserved when toggling WYSIWYG/Source several times, and the restored selection is visible.
• xwiki-source.js#Convert text selection between WYSIWYG and Source modes: JS unit test of selection mapping (collapsed caret, bold, links, macros, paragraphs); no headings or table cells.

Missing automated coverage

• Edit in-place a page with a paragraph, a heading and a table.
• Put a collapsed caret in a paragraph word, toggle Source and assert the caret position in the source text area.
• Select an entire word in the heading, toggle Source and assert the source selection is that word; toggle back and assert.
• Select an entire table cell value, toggle Source and assert the source selection is that value; toggle back and assert.

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

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