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GH-34641: [CI][Python] Mark test_scan on test_acero.py to require dataset (#34642)

### Rationale for this change

Some nightly builds were failing.

### What changes are included in this PR?

Correctly skip test if no dataset.

### Are these changes tested?

Yes, locally and will test the CI jobs

### Are there any user-facing changes?

No
* Closes: #34641

Authored-by: Raúl Cumplido <raulcumplido@gmail.com>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>