test/py: fs: add extended write operation test

In this commit and the following, test scripts for new filesystem
functionalities introduced by my patch set, "fs: fat: extend FAT write
operations," are provided.

In particular, this patch adds test cases for sub-directory write
and write with non-zero offset.

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