Revert "truncate debug.log on each test run"

This reverts commit 98043c68.

Because if every time `debug.log` is truncated,
developers have no way to see the previous ActiveRecord unit test results.
`debug.log` file can be easily reduced
by executing `$ touch /dev/null > debug.log` periodically.

activerecord/test/support/connection.rb

@@ -13,8 +13,7 @@ def self.connection_config
 
   def self.connect
     puts "Using #{connection_name}"
-    file = File.open('debug.log', 'w')
-    ActiveRecord::Model.logger = ActiveSupport::Logger.new(file)
+    ActiveRecord::Model.logger = ActiveSupport::Logger.new("debug.log")
     ActiveRecord::Model.configurations = connection_config
     ActiveRecord::Model.establish_connection 'arunit'
     ARUnit2Model.establish_connection 'arunit2'