ITS: test_preview_zoom: Reduce max zoom to match zoom range with test_zoom

Reduce max zoom to match zoom range with test_zoom. As zoom range is
reduce, reduce zoom steps from 100 to 50. This should reduce size of the
log.

Bug: 346845770
Test: test_preview_zoom.py
Change-Id: I990f1407fdfa027c3a58708b5ce9b532aafb2998