Add a regression test for a removed method in 6.1

This method has been moved in rails/rails@3cece0b
We do not include ActiveSupport::Testing::Assertion.

This resulted in failures when `perform_enqueued_jobs do` was called
from specs.

Method was added back in rails/rails#40780, and
was released in Rails 6.1.1

Author: benoittgt

spec/rspec/rails/matchers/active_job_spec.rb

@@ -702,4 +702,16 @@ def self.name; "LoggingJob"; end
       }.to raise_error(/expected to perform exactly 1 jobs, but performed 0/)
     end
   end
+
+  describe 'Active Job test helpers' do
+    include ActiveJob::TestHelper
+
+    it 'does not raise that "assert_nothing_raised" is undefined' do
+      expect {
+        perform_enqueued_jobs do
+          :foo
+        end
+      }.to_not raise_error
+    end
+  end
 end