tests/functional: let check clean directories of passed tests in default

Some tests consume lots of disk space and cause disk full (at least on
my box with 64GB SSD). This patch lets the check script clean
directories of passed tests in default. In addition, this patch adds a
new option -keep-passed. When this option is passed, the script keeps
directories of passed tests. This would be useful when we need to
check correctness of test scriptes themselves.

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