6653: modify slow test but keep coverage

Before the original changes were made at 0fa7b9730e56759e5f4bc3752143f5,
the expression added in this PR would also fail in the same way, raising
an assertion. And if the assertion were removed, this wrong expression
was obtained:

1 + O(x**2)

After the changes, the expression covers all added code and gives the
following result:

1 - x**2/2 + O(x**3)