tests/extmod/vfs_fat_more: Make skippable is uos is not available.

Fixes Zephyr tests.

tests/extmod/vfs_fat_more.py

@@ -1,10 +1,15 @@
 import sys
 import uerrno
 try:
+    try:
         import uos_vfs as uos
         open = uos.vfs_open
-except ImportError:
+    except ImportError:
         import uos
+except ImportError:
+    print("SKIP")
+    sys.exit()
+
 try:
     uos.VfsFat
 except AttributeError: