tests: fix post and pool version tests helpers.

* Fix the pool version SQS service to always be mocked before every
  test. Before we had to manually set it up before every test dealing
  with pool versions.

* Fix it so that we reconnect to the post/pool version databases before
  every test. Before using $ARCHIVE_DATABASE_URL to set the database url
  failed because environment variables weren't loaded by dotenv yet when
  connections were first established.

app/models/pool_version.rb

@@ -3,10 +3,14 @@ class PoolVersion < ApplicationRecord
   belongs_to :pool
 
   def self.enabled?
-    Danbooru.config.aws_sqs_archives_url.present?
+    Rails.env.test? || Danbooru.config.aws_sqs_archives_url.present?
   end
 
-  establish_connection (ENV["ARCHIVE_DATABASE_URL"] || "archive_#{Rails.env}".to_sym) if enabled?
+  def self.database_url
+    ENV["ARCHIVE_DATABASE_URL"] || "archive_#{Rails.env}".to_sym
+  end
+
+  establish_connection database_url if enabled?
 
   module SearchMethods
     def default_order

app/models/post_version.rb

@@ -9,7 +9,11 @@ class PostVersion < ApplicationRecord
     Rails.env.test? || Danbooru.config.aws_sqs_archives_url.present?
   end
 
-  establish_connection (ENV["ARCHIVE_DATABASE_URL"] || "archive_#{Rails.env}".to_sym) if enabled?
+  def self.database_url
+    ENV["ARCHIVE_DATABASE_URL"] || "archive_#{Rails.env}".to_sym
+  end
+
+  establish_connection database_url if enabled?
 
   def self.check_for_retry(msg)
     if msg =~ /can't get socket descriptor/ && msg =~ /post_versions/