common/renameat2: use mktemp(1) to create temporary directory

common/renameat2: use mktemp(1) to create temporary directory

Instead of using $TEST_DIR/$$, use the _correct_ way to create temporary
directories.  I've seen generic/626 failing with:

  QA output created by 626
  mkdir: cannot create directory '/media/test/471': File exists
  Silence is golden

which was likely due to another test that used the same directory name and
didn't do the clean-up.

Signed-off-by: Luís Henriques <lhenriques@suse.de>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>