Documenting duplicate named tests not supported in junit feature

What does this MR do?

• Adding documentation to clarify that when you have duplicate names/classes in your Junit test files, that the behaviour is undefined.
• Some page cleanup - wiki link no longer links to relevant list of junit supported languages. Removing note from artifacts:path example, as it seems more instructional than a note

Screenshots

Related issues

Author's checklist (required)

• Follow the Documentation Guidelines and Style Guide.
• If you have developer access or higher (for example, GitLab team members or Core Team members)
  • Apply the documentation label, plus:
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Do not add the feature, frontend, backend, ~"bug", or database labels if you are only updating documentation. These labels will cause the MR to be added to code verification QA issues.

When applicable:

• [-] Update the permissions table.
• [-] Link docs to and from the higher-level index page, plus other related docs where helpful.
• [-] Add GitLab's version history note(s).
• [-] Add the product tier badge.
• [-] Add/update the feature flag section.
• [-] If you're changing document headings, search doc/*, app/views/*, and ee/app/views/* for old headings replacing with the new ones to avoid broken anchors.

Review checklist

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1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

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