fix test_properties

Signed-off-by: Jinzhe Zeng <jinzhe.zeng@rutgers.edu>

tests/analysis/elasticity/test_stress.py

@@ -52,4 +52,4 @@ def test_properties(self):
             UserWarning, match="Tensor is not symmetric, information may be lost in Voigt conversion"
         ) as warns:
             _ = self.non_symm.voigt
-        assert len(warns) == 1
+        assert sum("Tensor is not symmetric, information may be lost in Voigt conversion" in str(warn) for warn in warns) == 1