modify 4 minute test so it takes 2 seconds seconds

Before original commit:
>>> series(cot(x**2 + x**3), x, 0, 1)
x**(-2) + O(x)

After original commit
>>> series(cot(x**2 + x**3), x, 0, 1)
x**(-2) - 1/x + 1 + O(x)

This simpler test failed in master before the original
changes but succeeds afterwards demonstrating that the
change worked. It also takes orders of magnitude less time
to compute.