Fix expected shape in matrix sum test

Updated the assertion in test_matrix_sum_argument to expect shape (1,) instead of (1, 1) when summing along axis 0. This aligns the test with the actual output of the sum operation.

tests/test_matrix_variable.py

@@ -185,7 +185,7 @@ def test_matrix_sum_argument():
 
     # Return a array when axis isn't None
     res = m.addMatrixVar((3, 1)).sum(axis=0)
-    assert isinstance(res, MatrixExpr) and res.shape == (1, 1)
+    assert isinstance(res, MatrixExpr) and res.shape == (1,)
 
     # compare the result of summing 2d array to a scalar with a scalar
     x = m.addMatrixVar((2, 3), "x", "I", ub=4)