Add missing tests for indexes

[itholic]

I found that there was not enough exception testing for the names property.

So I added tests for those exceptions,

And also added tests for some other code that isn't being tested yet.

It's very appreciate it if someone check it out when available. :)

Thanks.

[softagram-bot]

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Codecov Report

Merging #692 into master will increase coverage by 0.1%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #692      +/-   ##
==========================================
+ Coverage   93.54%   93.65%   +0.11%     
==========================================
  Files          32       32              
  Lines        5455     5455              
==========================================
+ Hits         5103     5109       +6     
+ Misses        352      346       -6

Impacted Files	Coverage Δ
databricks/koalas/indexes.py	97.19% <0%> (+5.6%)	⬆️

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