Add test coverage for settings form.

The last submitted patch, 4: simple_decoupled_preview-n3348364-4.patch, failed testing. View results →
- codesniffer_fixes.patch Interdiff of automated coding standards fixes only.

Looks good. Two items:
1. The test failure seems legit - shouldn't the "page" item have the value "uid" because that's what's saved via the submitForm() method just a few lines higher up?
2. I think the assertNonAdminsCannotAccessForm() code could be moved inline, it doesn't serve much purpose being split out like that when other code isn't split out similarly.

1. The build passes as is with "page" already having "uid"
   $this->assertEquals(['article' => 'uid', 'page' => 'uid'], $config->get('includes'));
2. Removed the function and moved code inline

LGTM!