run_in_isolation

# File activesupport/lib/active_support/testing/isolation.rb, line 111
        def run_in_isolation(&blk)
          require "tempfile"

          if ENV["ISOLATION_TEST"]
            proxy = ProxyTestResult.new
            retval = yield proxy
            File.open(ENV["ISOLATION_OUTPUT"], "w") do |file|
              file.puts [Marshal.dump([retval, proxy])].pack("m")
            end
            exit!
          else
            Tempfile.open("isolation") do |tmpfile|
              ENV["ISOLATION_TEST"]   = @method_name
              ENV["ISOLATION_OUTPUT"] = tmpfile.path

              load_paths = $-I.map {|p| "-I\"#{File.expand_path(p)}\"" }.join(" ")
              `#{Gem.ruby} #{load_paths} #{$0} #{ORIG_ARGV.join(" ")} -t\"#{self.class}\"`

              ENV.delete("ISOLATION_TEST")
              ENV.delete("ISOLATION_OUTPUT")

              return tmpfile.read.unpack("m")[0]
            end
          end
        end