task: #2999441 Convert or supplement TaxonomyTermIndentationTest with a javascript test

This MR converts TaxonomyTermIndentationTest from a Functional test into a FunctionalJavascript test, using the real tabledrag JavaScript instead of faking it.

Previously, the test set the hidden parent/depth form fields directly in PHP and submitted the form — which verified the form-processing logic, but never actually exercised the drag-and-drop JavaScript it was supposed to be testing.

What this MR does:

• Adds core/modules/taxonomy/tests/src/FunctionalJavascript/TaxonomyTermIndentationTest.php, which drives the actual tabledrag handle via its keyboard-accessible indent controls (the arrow-key equivalent of a mouse drag) to nest one term under another, then verifies:
  • the .js-indentation markup and hidden parent/depth fields update live in the DOM
  • the "You have unsaved changes" warning appears
  • the parent/child relationship is correctly persisted after saving
  • reversing the move removes the indentation and clears the parent relationship
• Removes core/modules/taxonomy/tests/src/Functional/TaxonomyTermIndentationTest.php, since it's fully superseded by the new test.

Closes #2999441