fix: add fixture for dataproc batch and better test cleanup

[leahecole]

Description

#8021 was similar but was throwing the Aborted error, this is giving a 409 AlreadyExists which was happening when it would retry but already find a batch there. I've added teardown logic to hopefully reduce the likelihood that an AlreadyExists happens.

Fixes #8254

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Checklist

• [x] I have followed Sample Guidelines from AUTHORING_GUIDE.MD
• [x] README is updated to include all relevant information
• [x] Tests pass: nox -s py-3.9 (see Test Environment Setup)
• [x] Lint pass: nox -s lint (see Test Environment Setup)
• [ ] These samples need a new API enabled in testing projects to pass (let us know which ones)
• [ ] These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• [ ] Please merge this PR for me once it is approved.
• [ ] This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample