Refactor test

This avoids leaking the class definition.

spec/faraday/middleware_registry_spec.rb

@@ -1,30 +1,31 @@
 # frozen_string_literal: true
 
-class CustomMiddleware < Faraday::Middleware
-end
-
 RSpec.describe Faraday::MiddlewareRegistry do
+  before do
+    stub_const('CustomMiddleware', custom_middleware_klass)
+  end
+  let(:custom_middleware_klass) { Class.new(Faraday::Middleware) }
   let(:dummy) { Class.new { extend Faraday::MiddlewareRegistry } }
 
   after { dummy.unregister_middleware(:custom) }
 
   it 'allows to register with constant' do
-    dummy.register_middleware(custom: CustomMiddleware)
-    expect(dummy.lookup_middleware(:custom)).to eq(CustomMiddleware)
+    dummy.register_middleware(custom: custom_middleware_klass)
+    expect(dummy.lookup_middleware(:custom)).to eq(custom_middleware_klass)
   end
 
   it 'allows to register with symbol' do
     dummy.register_middleware(custom: :CustomMiddleware)
-    expect(dummy.lookup_middleware(:custom)).to eq(CustomMiddleware)
+    expect(dummy.lookup_middleware(:custom)).to eq(custom_middleware_klass)
   end
 
   it 'allows to register with string' do
     dummy.register_middleware(custom: 'CustomMiddleware')
-    expect(dummy.lookup_middleware(:custom)).to eq(CustomMiddleware)
+    expect(dummy.lookup_middleware(:custom)).to eq(custom_middleware_klass)
   end
 
   it 'allows to register with Proc' do
-    dummy.register_middleware(custom: -> { CustomMiddleware })
-    expect(dummy.lookup_middleware(:custom)).to eq(CustomMiddleware)
+    dummy.register_middleware(custom: -> { custom_middleware_klass })
+    expect(dummy.lookup_middleware(:custom)).to eq(custom_middleware_klass)
   end
 end