After running the test suite, `df` used to report this:

  df: /tmp/TestReaddirTypeFixup090721515: Transport endpoint is not connected

To catch problems like this earlier, testMount() now reports failures to umount
cleanly, and catches the problem with TestReaddirTypeFixup:

  $ go test ./fs -run TestReaddirTypeFixup
  [...]
      mem_test.go:37: testMount: Unmount failed: /usr/bin/fusermount: failed to unmount /tmp/TestReaddirTypeFixup175482633: Device or resource busy

TestReaddirTypeFixup was missing ds.Close(), which was also added,
tests pass now and don't leave broken mounts behind.

Change-Id: Ib5d96c8531b11080cf8da526b917446388537956