Follow-up for #3060396: make CKEditor Widget JS tests conform to best practices, and harden them

While working on the port of the Entity Embed CKEditor Plugin's test coverage to core in #2940029: Add an input filter to display embedded Media entities, I spotted:

1. The copying of an existing file instead of using TestFileCreationTrait
2. Several unused test class properties
3. Inaccurate comments
4. Every test is calling drupalLogin($this->adminUser) — might as well do that in ::setUp() then :)
5. Some unnecessarily complex XPath-based element selection, converting them to CSS-based selectors simplifies the tests and makes them accessible to more people
6. Some missing "waits"
7. A few assertions that are too high-level and thus don't necessarily prove something is still working
8. Some typos