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)
remove print statements [skip ci]
Merge pull request #7945 from smichr/6653

modify tests while maintaining coverage
Raise TypeError instead of returning False, if we can't sympify 'other'

Unfortunately, we can't use _sympifyit decorator, because in py2 class
instances are compared by object identity ("address") if no other
comparison helpers are defined.

fixes #7833
XXX: use S.Zero in _infimum_key, if we can't return infimum