401: Add tests for 2d mode ratios

[John-Sharples]

New tests added for mode 2d ratio calculations

• Describe testing already performed for these changes:
  
  tests run locally
• Recommend testing for the reviewer(s) to perform, including the location of input datasets, and any additional instructions:
  
  NA
• Do these changes include sufficient documentation updates, ensuring that no errors or warnings exist in the build of the documentation? [Yes or No]
  Yes
• Do these changes include sufficient testing updates? [Yes or No]
  Yes
• Will this PR result in changes to the test suite? [Yes or No]
  
  If yes, describe the new output and/or changes to the existing output:
  
  Yes - more test coverage
• Do these changes introduce new SonarQube findings? [Yes or No]
  
  If yes, please describe:
  No
• Please complete this pull request review by [Fill in date].
  

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