After the changes to the `lang-none` styling in #41335, code snippets marked with
```
language="none" class="code-shell"
```
were being styled with the same foreground and background colours.
It turns out that most of these ought to be marked `language="sh"`
in which case the `code-shell` style became redundant and has been
removed.
Fixes#41984
PR Close#41986
This commit breaks up the testing document into nine total documents, with each document focusing on an existing section of the current testing documentation. There are no content changes.
PR Close#37957