Bugfix #3051 main_v12.0 unit test decouple install dir from repo

[georgemccabe]

These changes allow the unit tests to be run to test a MET installation that is found in another location besides the MET repository source code.

Expected Differences

• Do these changes introduce new tools, command line arguments, or configuration file options? [No]
  
  If yes, please describe:
  

• Do these changes modify the structure of existing or add new output data types (e.g. statistic line types or NetCDF variables)? [No]
  
  If yes, please describe:
  

Pull Request Testing

• Describe testing already performed for these changes:
  

Ran tests on seneca to test a conda installation of MET. I made sure that the files that are in the repository and not installed are found relative to the test_unit directory and files that should be installed are referenced relative to MET_BASE.

• Recommend testing for the reviewer(s) to perform, including the location of input datasets, and any additional instructions:
  

Ensure all automated tests pass

• Do these changes include sufficient documentation updates, ensuring that no errors or warnings exist in the build of the documentation? [Yes]

Removed references to MET_BUILD_BASE since we no longer require it to be set to run the tests

• Do these changes include sufficient testing updates? [Yes]

• Will this PR result in changes to the MET test suite? [No]
  
  If yes, describe the new output and/or changes to the existing output:
  

• Will this PR result in changes to existing METplus Use Cases? [No]
  
  If yes, create a new Update Truth METplus issue to describe them.

• Do these changes introduce new SonarQube findings? [No]
  
  If yes, please describe:

• Please complete this pull request review by 1/17/2025.
  

Pull Request Checklist

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• Complete the PR definition above.
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[JohnHalleyGotway]

I approve of these changes. The changes match what we already discussed, and I'm glad to see all the GHA tests passed. Thanks for wrapping this up.