run_in_isolation

# File activesupport/lib/active_support/testing/isolation.rb, line 36
        def run_in_isolation(&blk)
          read, write = IO.pipe
          read.binmode
          write.binmode

          pid = fork do
            read.close
            yield
            begin
              if error?
                failures.map! { |e|
                  begin
                    Marshal.dump e
                    e
                  rescue TypeError
                    ex = Exception.new e.message
                    ex.set_backtrace e.backtrace
                    Minitest::UnexpectedError.new ex
                  end
                }
              end
              result = Marshal.dump(self.dup)
            end

            write.puts [result].pack("m")
            exit!
          end

          write.close
          result = read.read
          Process.wait2(pid)
          return result.unpack("m")[0]
        end