use Dir.exist? instead of deprecated Dir.exists?

Author: y-yagi

railties/lib/rails/test_unit/runner.rb

@@ -63,7 +63,7 @@ def self.parse(args)
             options[:line] &&= options[:line].to_i
           else
             arg = arg.gsub(':', '')
-            if Dir.exists?("#{arg}")
+            if Dir.exist?("#{arg}")
               options[:patterns] << File.expand_path("#{arg}/**/*_test.rb")
             elsif File.file?(arg)
               options[:patterns] << File.expand_path(arg)