tests/extmod/vfs_fat: Add test for ilistdir of a non-existent directory.

tests/extmod/vfs_fat_ramdisk.py

@@ -93,3 +93,9 @@ uos.umount(vfs)
 
 vfs = uos.VfsFat(bdev)
 print(list(vfs.ilistdir(b"")))
+
+# list a non-existent directory
+try:
+    vfs.ilistdir(b"no_exist")
+except OSError as e:
+    print('ENOENT:', e.args[0] == uerrno.ENOENT)

tests/extmod/vfs_fat_ramdisk.py.exp

@@ -13,3 +13,4 @@ getcwd: /foo_dir
 True
 getcwd: /
 [(b'foo_file.txt', 32768, 0), (b'foo_dir', 16384, 0)]
+ENOENT: True