tests: Improve feature detection for VFS.

tests/extmod/vfs_fat_more.py

@@ -1,4 +1,3 @@
-import uerrno
 try:
     import uos
 except ImportError:

tests/extmod/vfs_userfs.py

@@ -1,9 +1,10 @@
 # test VFS functionality with a user-defined filesystem
 # also tests parts of uio.IOBase implementation
 
-import sys, uio
+import sys
 
 try:
+    import uio
     uio.IOBase
     import uos
     uos.mount