TST: add additional test cases for pivot_table with categorical data

[jorisvandenbossche]

xref #21252 (comment)

Add additional test cases for:

Using the same definition of df as you used in your test, columns/values is incorrect:

In [3]: pd.__version__
Out[3]: '0.23.0'

In [4]: df.pivot_table(columns='A', values='B')
Out[4]:
A  NaN  low
B  2.0  3.0

Similarly index/columns/values is incorrect:

In [5]: df['AA'] = df['A']

In [6]: df.pivot_table(index='A', columns='AA', values='B')
Out[6]:
AA   NaN  low
A
NaN  2.0  NaN
low  NaN  3.0

[uds5501]

@jorisvandenbossche if this issue is still open, I would like to take this one up!

[gfyoung]

@uds5501 : This is definitely still fair game. Go for it!