Fix FillValueTest failure in CTest

[hyungyukang]

This PR fixes the FillValueTest failure in CTests that appeared after merging the fill value PR (#428) and the vertical mixing tendency PR (#410). The test now initializes VertMix and Eos as required.

Checklist

• Linting
  • Pre-commit run locally
• Building
  • CMake build does not produce any new warnings from changes in this PR
• Testing
  • Add a comment to the PR titled Testing with the following:
    • Which machines CTest unit tests
      have been run on and indicate that are all passing.
    • Indicate "All tests passed" or document failing tests

[hyungyukang]

Testing

CTest unit tests

• Machine: Frontier
• Compiler: craygnu, craygnu-mphipcc
• Build type: Release
• Result: All tests passed

[katsmith133]

Testing

CTest unit tests

• Machine: PM-CPU, PM-GPU
• Compiler: gnu, gnugpu
• Build type: Release
• Result: All tests passed

[katsmith133]

Approving based upon visual inspection and testing done by @hyungyukang on Frontier and myself on PM.

[sbrus89]

All CTests pass on chrysalis (oneapi-ifx). Thanks to @alicebarthel for noticing and to hyungyukang and @brian-oneill for helping with the fix.