FIX: use `with pytest.raises` instead of `pytest.xfail`

tests/core/test_flags.py

@@ -313,20 +313,7 @@ def test_to_frame(data: Union[pd.DataFrame, dios.DictOfSeries, Dict[str, pd.Seri
 
 @pytest.mark.parametrize(
     "columns",
-    [
-        # ok
-        ["x", "y"],
-        pd.Index(["x", "y"]),
-        # expect to fail
-        pytest.param(
-            pd.Index(["x", "x"]), marks=pytest.mark.xfail(reason="not unique")
-        ),
-        pytest.param(pd.Index(["x"]), marks=pytest.mark.xfail(reason="wrong length")),
-        pytest.param(
-            pd.Index(["x", "x", "x"]),
-            marks=pytest.mark.xfail(reason="wrong length"),
-        ),
-    ],
+    [["x", "y"], pd.Index(["x", "y"])],
 )
 def test_columns_setter(columns):
     flags = Flags(
@@ -335,3 +322,21 @@ def test_columns_setter(columns):
     flags.columns = columns
     for c in columns:
         assert c in flags.columns
+
+
+@pytest.mark.parametrize(
+    "columns,err",
+    [
+        ("foooo", TypeError),  # cannot cast to Index
+        (pd.Index(["x", "x"]), TypeError),  # duplicates
+        (pd.Index([1, 2]), TypeError),  # not string
+        (pd.Index(["x", "y", "z"]), ValueError),  # wrong length
+        (pd.Index(["x"]), ValueError),  # wrong length
+    ],
+)
+def test_columns_setter_raises(columns, err):
+    flags = Flags(
+        {"a": pd.Series([1, 2], dtype=float), "b": pd.Series([1, 2], dtype=float)}
+    )
+    with pytest.raises(err):
+        flags.columns = columns