modify 4 minute test so it takes about 2 seconds

Before original commit:
>>> series(cot(x**4 + x**5), x, 0, 1)
O(x)

After original commit
>>> series(cot(x**4 + x**5), x, 0, 1)
x**(-4) - 1/x**3 + x**(-2) - 1/x + 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.

Since the time that the correction was made, another correction
was made that includes the "+1" term; that is now included in the
modified test, too.
Merge pull request #7751 from smichr/slow

modify test that took 4 minutes to compute