Fix random CI fail due to auto-updating timestamp

Example failure: https://buildkite.com/rails/rails/builds/68074#0fe7ca54-fcce-4a47-85db-a784275c8f51/1115-1125

Each time a job is serialized, `enqueued_at` is updated.  Thus, separate
serializations of the same job can have different `enqueued_at`
timestamps if the serializations do not occur within the same second.

activejob/test/cases/job_serialization_test.rb

@@ -22,6 +22,9 @@ class JobSerializationTest < ActiveSupport::TestCase
   end
 
   test "serialize and deserialize are symmetric" do
+    # Ensure `enqueued_at` does not change between serializations
+    freeze_time
+
     # Round trip a job in memory only
     h1 = HelloJob.new("Rafael")
     h2 = HelloJob.deserialize(h1.serialize)