Add test coverage for Drupal behavior attachment scope after HTMX swaps

Problem/Motivation

Follow-up of #3555916: Update to htmx 4

Nothing asserts the invariant that rewrite establishes. I grepped the
branch and found no test covering behavior attach scope.

I have two FunctionalJavascript tests that cover it:

testOuterHtmlSwapDoesNotAttachToDocument
testOutOfBandSwapAttachesBehaviors

They pass against the current head, and both fail if behaviorTarget() is made
to return a detached parent, so they discriminate rather than pass vacuously.
They need a small recorder added to the test_htmx behavior and two routes on
HtmxTestAttachmentsController.

Happy to push them to the merge request, or open a follow-up if you would
rather not expand scope now. No objection to RTBC either way.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet