tests/btrfs.c: add pid suffix to the created directory, remove it on exit

tests/btrfs.c: add pid suffix to the created directory, remove it on exit

If multiple tests are running simultaneously in the same directory,
conflicts (and test failures) may happen.

* tests/btrfs.c (path, dir_name_fmt, dir_name): New variables.
(btrfs_test_ino_path_ioctls): Use dir_name for printing directory name.
(rm_test_dir): New function.
(main): Generate dir_name, use it instead of fixed name, add rm_test_dir
as an atexit handler.