Fix (flash) Gemma prefix and enable tests

[danieldk]

What does this PR do?

While testing my exl2 branch, I noticed that Gemma models do not work due to using an incorrect prefix. Fix this, also enable tests using the Gemma-2B model.

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• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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[drbh]

LGTM! 🙏 Thank you!