forwardport #9388 rename/remove column preserves custom PK.
rename_column_test.rb -> columns_test.rb to reveal intent.

I renamed the test to better communicate it's intention. Since it also tests:
  - add_column
  - remove_column
  - change_column
There is no reason to call it rename_column_test.
Merge pull request #9389 from senny/forwardport_9388

Forwardport #9388 rename/remove column preserves custom PK.