Replace nccmp with xarray in test_remap_pp_components.py

[Copilot]

Describe your changes

Remove nccmp subprocess calls from two test files, replacing with native xarray comparisons.

fre/app/remap_pp_components/tests/test_remap_pp_components.py:

• nccmp -d → xr.testing.assert_equal for 3 data comparison tests
• Renamed test_nccmp_* → test_compare_*

fre/pp/tests/test_split_netcdf.py:

• nccmp -d --force → xr.testing.assert_equal in test_split_file_data
• nccmp -mg --force → xr.testing.assert_identical in test_split_file_metadata (covers both global attrs and variable metadata)

Both files use context managers for dataset cleanup:

with xr.open_dataset(path1, decode_cf=False, decode_times=False) as ds1, \
     xr.open_dataset(path2, decode_cf=False, decode_times=False) as ds2:
    xr.testing.assert_equal(ds1, ds2)

xarray is already a project dependency; no new dependencies added.

Issue ticket number and link (if applicable)

Checklist before requesting a review

• I ran my code
• I tried to make my code readable
• I tried to comment my code
• I wrote a new test, if applicable
• I wrote new instructions/documentation, if applicable
• I ran pytest and inspected it's output
• I ran pylint and attempted to implement some of it's feedback
• No print statements; all user-facing info uses logging module

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.12%. Comparing base (7e80492) to head (f7fe253).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #827   +/-   ##
=======================================
  Coverage   84.12%   84.12%           
=======================================
  Files          71       71           
  Lines        4944     4944           
=======================================
  Hits         4159     4159           
  Misses        785      785           

Flag	Coverage Δ
unittests	84.12% <ø> (ø)

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