Add cacheability tests

Problem/Motivation

🐛 Direct URL to media file entity does not work because relative URL does not pass URL path validation Fixed broke cacheability for pages with embedded links. Let's add test coverage to this module so it doesn't happen again.

Steps to reproduce

Install Linkit 6.1.1
Add page_cache or similar to the site, set cache max age to something other than off
Add a page with an embedded link
Notice the uncacheable page

Proposed resolution

Add a Functional test that checks the Cache-Control header, e.g
$this->assertSession()->responseHeaderEquals('Cache-Control', 'max-age=600, public');