New test is wrong:
```
In [1]: O(x**4*log(x)**4) in O(x**4) 
Out[1]: False

In [2]: (x**x).nseries(x, n=4)
Out[2]: 
                2    2       3    3                   
               x ⋅log (x)   x ⋅log (x)    ⎛ 4    4   ⎞
1 + x⋅log(x) + ────────── + ────────── + O⎝x ⋅log (x)⎠
                   2            6                     

In [3]: (x**x).nseries(x, n=5)
Out[3]: 
                2    2       3    3       4    4                   
               x ⋅log (x)   x ⋅log (x)   x ⋅log (x)    ⎛ 5    5   ⎞
1 + x⋅log(x) + ────────── + ────────── + ────────── + O⎝x ⋅log (x)⎠
                   2            6            24                    

In [4]: x**4*log(x)**4 in O(x**4)
Out[4]: False
```