tests/functional: unmount loopback devices before cleaning directories

Some tests (e.g. 011) make loopback devices and mount them in their
directories. These devices let cleaning fail. This patch let the check
script unmount them after succeeding.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>