tests: littlefs: fix false negative test_lfs_basic

Fixes the false negative littlefs.test_lfs_basic test that can occur
if the small_partition size > 64KB, and for
platforms with CONFIG_FS_LITTLEFS_PROG_SIZE > 64 (e.g. lpc55s, mcxn).

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