Limit roulette for docs changes to the most specific codeowners

What does this MR do and why?

Resolves #1137 (closed).

The first commit simplifies one of the test files and makes it describe the behavior of the app better.

• Improve the textual descriptions of test cases.
• Individual tests build up test state more locally. This reduces coupling, shared mutable state, and makes the test control flow more linear.
• Merges two tests that were specifying the same property of the system under test.
• Reduces the overall complexity and line count of the test file without reducing test coverage.

The second commit adds the feature to perform reviewer roulette only within the most specific code owners group instead of between all eligible approvers.

Expected impact & dry-runs

This change affects how reviewers for MR containing documentation changes are assigned.

• Before: It determines all eligible code owners and assigns a random person from that list as the reviewer.
• After: It determines the code owners from the most specific section in the CODEOWNERS file and assigns a random person from that list as the reviewer.

Action items

• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review