Replace failing t/tar-race.t with t/tar-missing.t

RT 95838: Replace failing t/tar-race.t, which intended to check test the
facilities for handling missing real files mapped into a Filesys::POSIX
filesystem during tar() time, with t/tar-missing.t; changes:

    * Rather than fork()ing a subprocess to perform the archiving, archive
      to a file handle opened for /dev/null

    * Use map() to bring a file in from the real filesystem into a fake one,
      and remove the referred file from $fs immediately prior to tar() to
      ensure desired condition is triggered deterministically