Flip test on the sign of the wavecorr correction

[gbrammer]

The signs of source_xpos and dwave_step should be the same. The sign of the wavelength correction was incorrect in earlier versions of the jwst pipeline, perhaps jwst<=1.15. However, they're now correct in jwst==1.16.

Other changes here made to pass tests with jwst==1.16.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 47.14286% with 37 lines in your changes missing coverage. Please review.

Project coverage is 67.20%. Comparing base (5b887ff) to head (95e8c2f).

Files with missing lines	Patch %	Lines
msaexp/slit_combine.py	15.78%	32 Missing ⚠️
msaexp/tests/test_utils.py	86.66%	2 Missing ⚠️
msaexp/tests/test_combine.py	75.00%	1 Missing ⚠️
msaexp/tests/test_drizzle.py	85.71%	1 Missing ⚠️
msaexp/tests/test_spectra.py	83.33%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #80      +/-   ##
==========================================
- Coverage   67.47%   67.20%   -0.27%     
==========================================
  Files          16       16              
  Lines        7092     7151      +59     
==========================================
+ Hits         4785     4806      +21     
- Misses       2307     2345      +38     

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