[tests] linkcheck: add test coverage for behaviour of a locally-existing file.

[jayaddison]

Feature or Bugfix

• Feature

Purpose

• Adds test coverage to demonstrate how the linkcheck builder currently behaves with regard to local files that exist and are referenced in documentation.

Detail

• Assert that the conf.py file from the test-linkcheck testroot is considered a working link.

Relates

• Guardrail against breaking changes in [tests] linkcheck: bind each test HTTP server to a unique port per-testcase. #12126.

[picnixz]

Thank you!

[jayaddison]

Thanks!