Fix order walkthrough for iso uniqueness

[AlistairNorman]

Summary

Alternatively I also considered adding initialize_with { Spree::Country.find_or_initialize_by(iso:) } to the factory which seems like a broader solution to making sure iso uniqueness doesn't break tests but I find initialize_with results in some very unintuitive behaviour so I opted to avoid it.

If anyone wants to see the test failures, they can see this run. Not all of the failures are from the iso issue but a bunch of them are.

Checklist

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.66%. Comparing base (0994f63) to head (75c5aeb).
⚠️ Report is 16 commits behind head on main.

Files with missing lines	Patch %	Lines
...ore/lib/spree/testing_support/order_walkthrough.rb	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6432   +/-   ##
=======================================
  Coverage   89.66%   89.66%           
=======================================
  Files         990      990           
  Lines       20798    20792    -6     
=======================================
- Hits        18649    18644    -5     
+ Misses       2149     2148    -1     

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