Improve custom formatter definition inside the spec

spec/faraday/response/logger_spec.rb

@@ -67,8 +67,8 @@
   end
 
   context 'with custom formatter' do
-    before do
-      class CustomFormatter < Faraday::Logging::Formatter
+    let(:formatter_class) do
+      Class.new(Faraday::Logging::Formatter) do
         def initialize(*args)
           super
         end
@@ -83,11 +83,7 @@ def response(_env)
       end
     end
 
-    after do
-      Object.send(:remove_const, :CustomFormatter)
-    end
-
-    let(:logger_options) { { formatter: CustomFormatter } }
+    let(:logger_options) { { formatter: formatter_class } }
 
     it 'logs with custom formatter' do
       conn.get '/hello'