Use i18n translations instead of strings in tests

What does this MR do and why?

Describe in detail what your merge request does and why.

Replace content strings in tests with i18n references as suggested in the MR review comment

Cannot be merged until both of the following are merged:

• Add Edit Timeline Event Functionality !95061 (merged)
• Refactor Timeline Event Form !94899 (merged)

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MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.