CI/CD, Unit Testing, and WCRP Compliance for CMIP6Plus

[ilaflott]

Description: Ensure the new era integrations do not break existing pipelines and pass strict compliance.

• Write pytest functions in fremor/tests/ running a mock dataset against the CMIP6Plus CV and mip-cmor-tables to cover new code and cases as-needed
• Investigate cc-plugin-wcrp support for wcrp_cmip6plus.
• Update .github/workflows/wcrp_compliance_check.yml with a testing matrix to categorize CMIP6, CMIP6Plus, and CMIP7 outputs.