revert to using os.mount, etc. instead of vfs.mount

Author: dhalbert

tests/run-natmodtests.py

@@ -28,7 +28,8 @@
 
 # Code to allow a target MicroPython to import an .mpy from RAM
 injected_import_hook_code = """\
-import sys, io, vfs
+# CIRCUITPY-CHANGE: no vfs, but still have os
+import sys, io, os
 class __File(io.IOBase):
   def __init__(self):
     self.off = 0