tests: fix deadlock during artist ban test.

The artist ban tests deadlocked because of a weird interaction between
threads and database transactions when tagging posts in parallel. Add a
hack to work around it.

app/models/application_record.rb

@@ -168,7 +168,7 @@ class ApplicationRecord < ActiveRecord::Base
 
   concerning :ConcurrencyMethods do
     class_methods do
-      def parallel_each(batch_size: 1000, in_processes: 4, in_threads: nil)
+      def parallel_each(batch_size: 1000, in_processes: 4, in_threads: nil, &block)
         # XXX Use threads in testing because processes can't see each other's
         # database transactions.
         if Rails.env.test?
@@ -176,6 +176,9 @@ class ApplicationRecord < ActiveRecord::Base
           in_threads = 2
         end
 
+        # XXX Threads deadlock during tests if a transaction is open; do a non-parallel each.
+        return find_each(&block) if Rails.env.test? && connection.transaction_open?
+
         current_user = CurrentUser.user
         current_ip = CurrentUser.ip_addr