Add missing tests for groupby

[itholic]

Added missing tests or tests that weren't running properly for groupby.

I would be grateful if someone could check when possible.

Thanks.

[softagram-bot]

Softagram Impact Report for pull/709 (head commit: d1ac086)

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[codecov-io]

Codecov Report

Merging #709 into master will increase coverage by 0.14%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #709      +/-   ##
==========================================
+ Coverage   93.87%   94.01%   +0.14%     
==========================================
  Files          32       32              
  Lines        5548     5548              
==========================================
+ Hits         5208     5216       +8     
+ Misses        340      332       -8

Impacted Files	Coverage Δ
databricks/koalas/groupby.py	90.11% <0%> (+1.83%)	⬆️

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[HyukjinKwon]

@itholic, you can show the missing coverage next time with the screenshot like you did before.

[itholic]

@HyukjinKwon, Oh I was worried that maybe it would be too much information. But I will upload the screen together in the future as you have advised me. Thank you for your advice!