Improve Matrix Content (mxc://) URIs in tests

[ahmadkadri]

:heavy_check_mark: Checklist

• [ ] A changeset describing the change and affected packages (more info).
• [ ] Added or updated documentation.
• [ ] Tests for new functionality and regression tests for bug fixes.
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• [x] All your commits have a Signed-off-by line in the message (more info).

[changeset-bot[bot]]

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Latest commit: 86057b1c6a2f96de44e3b3317844b23b9e905b2c

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