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)
improved docstring for apart
remove print statements [skip ci]
7961: watch for rounding corner case
Merge pull request #7945 from smichr/6653

modify tests while maintaining coverage
Merge pull request #7905 from ClemensFMN/apart_docstring

improved docstring for apart in sympy/polys/partfrac.py
7963: Ray respects 2nd and 3rd quad angles
Merge pull request #7962 from smichr/7961

7961: watch for rounding corner case
Return False if Ellipses don't intersect
use Piecewise for Ray creation from angle
Merge pull request #7967 from MridulS/issue_7939

Return False if Ellipses don't intersect
Merge pull request #7964 from smichr/7963

7963: Ray respects 2nd and 3rd quad angles