skip snapshot tests where not present

Author: tswast

tests/unit/core/sql/test_ml.py

@@ -12,8 +12,12 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import pytest
+
 import bigframes.core.sql.ml
 
+pytest.importorskip("pytest_snapshot")
+
 
 def test_create_model_basic(snapshot):
     sql = bigframes.core.sql.ml.create_model_ddl(