Adapt to latest SSF changes

[waiting-for-dev]

Summary

After latest changes on SSF (see https://github.com/solidusio/solidus_starter_frontend/pull/327), we need to:

• Scope the SystemHelper module within SolidusStarterFrontend.
• Rename the RSpec helper to solidus_starter_frontend_spec_helper

Closes #234

Checklist

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[codecov[bot]]

Codecov Report

Merging #235 (2828b62) into master (61684eb) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #235   +/-   ##
=======================================
  Coverage   95.51%   95.51%           
=======================================
  Files          23       23           
  Lines         424      424           
=======================================
  Hits          405      405           
  Misses         19       19           

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