Make PG deadlock error more deterministic

We've seen occasional Travis failures mentioning deadlocks. I think
they're escaping from this test.

activerecord/test/cases/adapters/postgresql/transaction_test.rb

 require "cases/helper"
 require "support/connection_helper"
+require "concurrent/atomic/cyclic_barrier"
 
 module ActiveRecord
   class PostgresqlTransactionTest < ActiveRecord::PostgreSQLTestCase
@@ -61,26 +62,28 @@ class Sample < ActiveRecord::Base
     test "raises Deadlocked when a deadlock is encountered" do
       with_warning_suppression do
         assert_raises(ActiveRecord::Deadlocked) do
+          barrier = Concurrent::CyclicBarrier.new(2)
+
           s1 = Sample.create value: 1
           s2 = Sample.create value: 2
 
           thread = Thread.new do
             Sample.transaction do
               s1.lock!
-              sleep 1
+              barrier.wait
               s2.update_attributes value: 1
             end
           end
 
-          sleep 0.5
-
-          Sample.transaction do
-            s2.lock!
-            sleep 1
-            s1.update_attributes value: 2
+          begin
+            Sample.transaction do
+              s2.lock!
+              barrier.wait
+              s1.update_attributes value: 2
+            end
+          ensure
+            thread.join
           end
-
-          thread.join
         end
       end
     end