Added plurals tests for DutchLocale

[erwinmintiens]

Pull Request Checklist

Thank you for taking the time to improve Arrow! Before submitting your pull request, please check all appropriate boxes:

• 🧪 Added tests for changed code.
• 🛠️ All tests pass when run locally (run tox or make test to find out!).
• 🧹 All linting checks pass when run locally (run tox -e lint or make lint to find out!).
• 📚 Updated documentation for changed code.
• ⏩ Code is up-to-date with the master branch.

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Description of Changes

Added plural testcases for the DutchLocale class. Relates to #1022

[codecov]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage 👍

Coverage data is based on head (9c5f0ce) compared to base (1bba2a8).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #1132   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            9         9           
  Lines         2318      2318           
  Branches       492       492           
=========================================
  Hits          2318      2318           

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[krisfremen]

@erwinmintiens thanks for improving arrow!