tests: littlefs: fix test_lfs_basic for medium and large partitions

Fixes failed test_lfs_basic for medium partition > 960KB
and for large partition > 3MB.
Flashes with large erase size require larger test partition sizes
(e.g. s26ks512s0 has 256KB erase size).

Signed-off-by: Andrej Butok <andrey.butok@nxp.com>