test: fix warning about classes already defined by another spec

spec/support/shared_examples_for_jobs.rb

@@ -1,11 +1,17 @@
 RSpec.shared_examples 'a job retrying transient errors' do |job_class = described_class|
-  context 'when a transient network error is raised' do
-    ExconErrorJob = Class.new(job_class) do
-      def perform
-        raise Excon::Error::InternalServerError, 'msg'
-      end
+  ExconErrorJob = Class.new(job_class) do
+    def perform
+      raise Excon::Error::InternalServerError, 'msg'
     end
+  end if !defined?(ExconErrorJob)
 
+  StandardErrorJob = Class.new(job_class) do
+    def perform
+      raise StandardError
+    end
+  end if !defined?(StandardErrorJob)
+
+  context 'when a transient network error is raised' do
     it 'makes 5 attempts before raising the exception up' do
       assert_performed_jobs 5 do
         ExconErrorJob.perform_later rescue Excon::Error::InternalServerError
@@ -14,12 +20,6 @@ RSpec.shared_examples 'a job retrying transient errors' do |job_class = describe
   end
 
   context 'when another type of error is raised' do
-    StandardErrorJob = Class.new(job_class) do
-      def perform
-        raise StandardError
-      end
-    end
-
     it 'makes only 1 attempt before raising the exception up' do
       assert_performed_jobs 1 do
         StandardErrorJob.perform_later rescue StandardError