Assert that installing the module changes nothing but its own endpoints

Problem/Motivation

Inertness outside the module's own endpoints is one of the guarantees this module has to make. It is currently a claim about code that we audited by reading. A claim of that shape is only worth what a test says, because the next person to add a path processor or an event subscriber will not have read the audit.

Proposed resolution

• Functional test on a site with an article type, an alias, a menu, a view page, a contact form and two languages.
• Record the responses of those pages with the module uninstalled, install it, record again, and assert the pages are unchanged — status, redirects, and the parts of the markup that path handling affects.
• Assert the route collection differs only by routes this module registers.
• Assert the same after enabling every optional integration the module ships.

Remaining tasks

Everything. The audit issue produces the list of things that will fail this test first.