test/py: test_fs: add tests for creating/deleting many files

# This is actually a resent patch of
# [1] https://lists.denx.de/pipermail/u-boot/2019-May/369170.html



Two test cases are added under test_fs_ext:
    test case 10: for root directory
    test case 11: for non-root directory

Those will verify a behavior fixed by the commits related to
root directory
("fs: fat: allocate a new cluster for root directory of fat32" and
"fs: fat: flush a directory cluster properly"), and focus on
handling long-file-name directory entries under a directory.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>