TST: fix and xfail test for 19027

Author: toobaz

pandas/tests/frame/test_mutate_columns.py

@@ -174,6 +174,7 @@ def test_delitem(self):
         del self.frame['A']
         assert 'A' not in self.frame
 
+    @pytest.mark.xfail(reason='GH 19027 (x in pd.MultiIndex.drop(x))')
     def test_delitem_multiindex(self):
         midx = MultiIndex.from_product([['A', 'B'], [1, 2]])
         df = DataFrame(np.random.randn(4, 4), columns=midx)
@@ -195,7 +196,7 @@ def test_delitem_multiindex(self):
 
         # xref: https://github.com/pandas-dev/pandas/issues/2770
         # the 'A' is STILL in the columns!
-        assert 'A' in df.columns
+        assert 'A' not in df.columns
         with pytest.raises(KeyError):
             del df['A']