Use equivalent `__FILE__`

tool/lib/core_assertions.rb

@@ -284,7 +284,7 @@ module Test
         src = <<eom
 # -*- coding: #{line += __LINE__; src.encoding}; -*-
 BEGIN {
-  require "test/unit";include Test::Unit::Assertions;require #{(__dir__ + "/core_assertions").dump};include Test::Unit::CoreAssertions
+  require "test/unit";include Test::Unit::Assertions;require #{__FILE__.dump};include Test::Unit::CoreAssertions
   separated_runner #{res_c&.fileno}
 }
 #{line -= __LINE__; src}