Firstly, the assertion message should have been inverted:
See https://www.drupal.org/project/coding_standards/issues/2057905#comment-9470963

Secondly, we avoid assertion messages when possible. People can look up the line number on fail.

Why?
We are going to enable 'block' module as a dependency of oe_whitelabel_helper.
As a consequence, all block placements from config/optional will be installed in the test.
At least one of them requires sparql.

Changes:
- Set up sparql in base test class.
- Let CorporateHeaderLogosTest also inherit from the base class.

Changes:
- Reorder imports.
- Change line breaks and blank lines.
- Change doc comments on test methods.
- Change inline comments.
- Change order of assertions a little.

Rationale for inline comments and line breaks:
- When rendering the full page, inline comments are somewhat helpful, but not too many.
- Blank lines are nice for structuring content.
- The teaser view mode is simple enough that we don't need inline comments.
- The existing comments in teaser view mode were wrong anyway, because they mentioned "content banner".