Merge pull request rails#28199 from kirs/ar-test-connection

Handle non-existing $ARCONN

Author: matthewd

activerecord/test/support/connection.rb

@@ -10,7 +10,10 @@ def self.connection_name
   end
 
   def self.connection_config
-    config["connections"][connection_name]
+    config.fetch("connections").fetch(connection_name) do
+      puts "Connection #{connection_name.inspect} not found. Available connections: #{config['connections'].keys.join(', ')}"
+      exit 1
+    end
   end
 
   def self.connect