test/py: convert fs-test.sh to pytest

In this commit, the same set of test cases as in test/fs/fs-test.sh
is provided using pytest framework.
Actually, fs-test.sh provides three variants:"sb" (sb command), "nonfs"
(fatxx and etc.) and "fs" (hostfs), and this patch currently supports
only "nonfs" variant; So it is not a replacement of fs-test.sh for now.

Simple usage:
  $ py.test test/py/tests/test_fs [<other options>]

You may also specify filesystem types to be tested:
  $ py.test test/py/tests/test_fs --fs-type fat32 [<other options>]

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Signed-off-by: Alexander Graf <agraf@suse.de>