Remove outdated comment about integration test fixture inheritance

[Copilot]

Addresses feedback from #2442 to remove a misleading comment in test/test_helper.rb.

Changes

• Removed outdated comment claiming integration tests must declare fixtures explicitly
• Comment was incorrect: ActionDispatch::IntegrationTest inherits from ActiveSupport::TestCase, so integration tests automatically inherit the fixtures :all declaration

class ActiveSupport::TestCase
  fixtures :all  # Integration tests inherit this
  # ...
end

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