chore: Extract snapshot tests to different nox session

[TrevorBergeron]

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
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• Appropriate docs were updated (if necessary)

Fixes #<issue_number_goes_here> 🦕

[chelsea-lin]

I would like to keep the snapshot tests as part of the presubmit.

[chalmerlowe]

Migration Notice: This library is moving to the google-cloud-python monorepo soon.

We closed this PR due to inactivity to ensure a clean migration. Please re-open this work in the new monorepo once the migration is complete!