Add test coverage for specreduce error propogation

[cshanahan1]

(Note: uncertainties will be available in 1.8.0 which isn't out yet, so this has a minimum version check and tests can be run locally with specreduce dev)

This PR adds test coverage for propagation of uncertainties through specreduce background and extraction steps, which was recently implemented in specreduce.

It turns out that this works right out of the box, and the returned background/extractions already have uncertainties attached, so this is just adding test coverage for the 2d case. The tests here are identical to the tests in specreduce, but go through the plugin to ensure the results are the same.

(I also snuck in a change to switch one of the existing tests for spectral extraction unit conversion from specviz2d to deconfigged)

[rosteen]

I confirmed that the new tests pass locally with dev specreduce, approved.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.61%. Comparing base (50558dd) to head (f6fda92).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4047      +/-   ##
==========================================
- Coverage   84.61%   84.61%   -0.01%     
==========================================
  Files         202      202              
  Lines       28748    28748              
==========================================
- Hits        24326    24325       -1     
- Misses       4422     4423       +1     

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