Improve StableDecoupledTest in the 9.0.x branch

Problem/Motivation

StableDecoupledTest is now gone from 9.1.x branch.
However, it'll remain in the 9.0.x branch.
At #3138652: Remove StableDecoupledTest → I raised some issues with it, based on my experience from #3033734: Horizontal scrollbar on /admin/modules → :

A) $this->assertEmpty($files_inherited_from_stable); The default PHPUnit message when this assertion fails is just "Failed asserting that an array is empty." In #3033734 I had to add some debugging code to the test when running it locally to confirm system.admin.css was indeed the problem. It'd be really nice if the test output told us that directly.

B) If this test is going to fail (by design) whenever stable diverges from core markup, can we add some more comments to help folks understand how to properly fix the test in this (potentially common) case of failure? I.e. make it more obvious that 3033734.49_51.interdiff.txt is the intended solution whenever stable and core intentionally diverge?

C) There are a handful of typos in the comments.

Proposed resolution

Fix the things.

Remaining tasks

2. Reviews.
3. RTBC.
4. Commit to 9.0.x.

User interface changes

N/A

API changes

N/A

Data model changes

N/A

Release notes snippet

N/A