Configure Rails 7.1's test app separately

[st0012]

Rails 7.1 stores the error reporter directly under the ActiveSupport class (PR: rails/rails#46029). So we need to make sure the subscriber is not subscribed unexpectedly before any tests.

And because both the change and the unsubscribe API is introduced in Rails 7.1, we should start having a dedicated test setup for it now.

[codecov-commenter]

Codecov Report

Base: 98.40% // Head: 98.38% // Decreases project coverage by -0.02% ⚠️

Coverage data is based on head (481358a) compared to base (ae13d7d).
Patch coverage: 25.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1901      +/-   ##
==========================================
- Coverage   98.40%   98.38%   -0.03%     
==========================================
  Files         148      148              
  Lines        8891     8893       +2     
==========================================
  Hits         8749     8749              
- Misses        142      144       +2     

Impacted Files	Coverage Δ
sentry-rails/spec/dummy/test_rails_app/app.rb	89.70% <25.00%> (-2.72%)	⬇️

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[st0012]

ping @sl0thentr0py