Some ".deleted" logic was in place, but did not work
properly when the Inode was in the root directory.

Fix that by introducing the `found` variable and
add a test that verifies that it works.

Also, return `.go-fuse.$RANDOM/deleted` to make it
very unlikely that the placeholder name matches an
actual file or directory. Introducing the `/deleted`
subdir makes sure operations creating a file or
directory fail reliably with ENOENT.

Tests now look like this:

  $ go test ./fs -run TestPosix/RenameOpenDir -count 1  -v
  === RUN   TestPosix
  === RUN   TestPosix/RenameOpenDir
  [...]
  --- PASS: TestPosix (0.01s)
      --- SKIP: TestPosix/RenameOpenDir (0.01s)
          test.go:383: Fstat failed: no such file or directory. Known limitation - see https://github.com/hanwen/go-fuse/issues/55
  PASS
  ok    github.com/hanwen/go-fuse/v2/fs 0.014s

Change-Id: I2eb6fd48a11df543c9b7daf62647cb9d8a892568