Change the Messaging topic test to avoid accidental messages

[a-maurice]

Description

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

Change the Messaging topic test to subscribe, send, and unsubscribe in the same test. This is to prevent accidentally receiving a topic test while running a different test. Ideally it would use a more unique test topic, but creating a test topic takes some time after creation before it can be used to send messages.

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Testing

Describe how you've tested these changes.

https://github.com/firebase/firebase-unity-sdk/actions/runs/5604605972

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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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