Change how Messaging testapp gets registration token

[a-maurice]

Description

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

The OnTokenReceived callback is only called for new Messaging Tokens, so if the App has a cached token, it would not receive a new token, and thus the tests would timeout while waiting for it. Change the tests to use GetTokenAsync if necessary to get a token, and throw exceptions if the token is still missing.

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Testing

Describe how you've tested these changes.

https://github.com/firebase/firebase-unity-sdk/actions/runs/6028673810

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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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