Clarify JUnit XML format specification in unit test reports doc

What does this MR do and why?

This MR updates the unit test reports documentation to clarify which XML elements GitLab parses and how they map to the UI.

Changes

• Adds a table that describes the parsed JUnit XML elements and attributes.
• Clarifies that testcase classname becomes the suite name, not testsuite name.
• Adds an XML example with mapping to GitLab UI elements.

🧪 Review app

https://docs.gitlab.com/upstream-review-mr-ee-194388/ci/testing/unit_test_reports/#junit-xml-format-specification

📚 References

• Resolves Clarify JUnit XML format (#299086 - closed)
• Relates to FY26Q2 Stage Lead planning issue: Verify (gitlab-org/technical-writing/team-tasks#1305 - closed)
• JUnit parser code: https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/ci/parsers/test/junit.rb

Screenshots or screen recordings

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