Add extra test to check halfpoint

Co-authored-by: Chris Havlin <chris.havlin@gmail.com>

Author: cphyc

yt/data_objects/tests/test_time_series.py

@@ -148,3 +148,6 @@ def test_get_by_key(FakeDataset, fake_datasets):
 
     with pytest.raises(ValueError):
         ts.get_by_redshift(1000, tolerance=0.1)
+        
+    assert sfile_list[0] == ts.get_by_redshift(1/2, prefer='smaller').filename
+    assert sfile_list[1] == ts.get_by_redshift(1/2, prefer='larger').filename