test: refactoring: improved test output file handling

 * test/base.vim:
  * added parameter 'redir_overwrite_flag' to several redir functions;
   * made "append" the default (for backward compatibility);

 * test/runutil.vim:
  * improved test output file handling:
   * test output file will be re-created for each test group, so that
      output in incompatible formats is not mixed in the same file;
   * added 'FIXME:' entries describing more things that need to be done
      later;