Add a workaround for invalid test logs

[a-maurice]

Description

Provide details of the change, and generalize the change in the PR title above.

Sometimes the Unity editor test logs have an invalid character for utf-8, which causes errors. Add some error handling, replacing any bad characters discovered.

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Testing

Describe how you've tested these changes.

https://github.com/firebase/firebase-unity-sdk/actions/runs/9182014476

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Type of Change

Place an x the applicable box:

• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

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