Make SolidusAdmin's testing support code require-able

[mamhoff]

Summary

I need to move some of SolidusAdmin's specs into
solidus_legacy_promotions. When doing that, it's necessary to require these, so it'd be nice if they lived in lib/.

Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 96.42857% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 88.93%. Comparing base (1f42077) to head (4101d91).
Report is 1 commits behind head on main.

Files	Patch %	Lines
...b/solidus_admin/testing_support/feature_helpers.rb	94.44%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5700      +/-   ##
==========================================
+ Coverage   88.92%   88.93%   +0.01%     
==========================================
  Files         700      702       +2     
  Lines       16653    16681      +28     
==========================================
+ Hits        14808    14835      +27     
- Misses       1845     1846       +1     

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