Flaky test: MoveFileFromLowerLayer

Observed in https://travis-ci.org/containerd/aufs/builds/309042106 (merge commit for https://github.com/containerd/aufs/pull/5. PR itself was green)

```
--- FAIL: TestAufs/MoveFileFromLowerLayer (0.06s)
testsuite.go:805: Check snapshots failed: invalid argument
failed to mount
github.com/containerd/containerd/snapshot/testsuite.applyToMounts
/home/travis/gopath/src/github.com/containerd/containerd/snapshot/testsuite/helpers.go:24
github.com/containerd/containerd/snapshot/testsuite.createSnapshot
/home/travis/gopath/src/github.com/containerd/containerd/snapshot/testsuite/helpers.go:46
github.com/containerd/containerd/snapshot/testsuite.checkSnapshots
/home/travis/gopath/src/github.com/containerd/containerd/snapshot/testsuite/helpers.go:107
github.com/containerd/containerd/snapshot/testsuite.checkFileFromLowerLayer
/home/travis/gopath/src/github.com/containerd/containerd/snapshot/testsuite/testsuite.go:804
github.com/containerd/containerd/snapshot/testsuite.makeTest.func1
/home/travis/gopath/src/github.com/containerd/containerd/snapshot/testsuite/testsuite.go:90
testing.tRunner
/home/travis/.gimme/versions/go1.9.2.linux.amd64/src/testing/testing.go:746
runtime.goexit
/home/travis/.gimme/versions/go1.9.2.linux.amd64/src/runtime/asm_amd64.s:2337
failed to apply
github.com/containerd/containerd/snapshot/testsuite.createSnapshot
/home/travis/gopath/src/github.com/containerd/containerd/snapshot/testsuite/helpers.go:47
github.com/containerd/containerd/snapshot/testsuite.checkSnapshots
/home/travis/gopath/src/github.com/containerd/containerd/snapshot/testsuite/helpers.go:107
github.com/containerd/containerd/snapshot/testsuite.checkFileFromLowerLayer
/home/travis/gopath/src/github.com/containerd/containerd/snapshot/testsuite/testsuite.go:804
github.com/containerd/containerd/snapshot/testsuite.makeTest.func1
/home/travis/gopath/src/github.com/containerd/containerd/snapshot/testsuite/testsuite.go:90
testing.tRunner
/home/travis/.gimme/versions/go1.9.2.linux.amd64/src/testing/testing.go:746
runtime.goexit
/home/travis/.gimme/versions/go1.9.2.linux.amd64/src/runtime/asm_amd64.s:2337
failed to create snapshot 2
github.com/containerd/containerd/snapshot/testsuite.checkSnapshots
/home/travis/gopath/src/github.com/containerd/containerd/snapshot/testsuite/helpers.go:109
github.com/containerd/containerd/snapshot/testsuite.checkFileFromLowerLayer
/home/travis/gopath/src/github.com/containerd/containerd/snapshot/testsuite/testsuite.go:804
github.com/containerd/containerd/snapshot/testsuite.makeTest.func1
/home/travis/gopath/src/github.com/containerd/containerd/snapshot/testsuite/testsuite.go:90
testing.tRunner
/home/travis/.gimme/versions/go1.9.2.linux.amd64/src/testing/testing.go:746
runtime.goexit
/home/travis/.gimme/versions/go1.9.2.linux.amd64/src/runtime/asm_amd64.s:2337
helpers.go:52: drwx------ 4096 /tmp/snapshot-suite-Aufs-932587849
helpers.go:52: drwxr-xr-x 4096 /tmp/snapshot-suite-Aufs-932587849/root
helpers.go:50: -rw------- 65536 /tmp/snapshot-suite-Aufs-932587849/root/metadata.db [ "\x00\x00\x00\x00\x00\x00\x00\x00\x04\x00\x00\x00\x00\x00\x00\x00\xed\xda\f\xed\x02\x00\x00\x00\x00\x10\x00\x00\x00\x00\x00\x00\a\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\b\x00\x00\x00\x00\x00\x00\x00\f\x00\x00\x00\x00\x00\x00\x00" ...]
helpers.go:52: drwx------ 4096 /tmp/snapshot-suite-Aufs-932587849/root/snapshots
helpers.go:52: drwx------ 4096 /tmp/snapshot-suite-Aufs-932587849/root/snapshots/1
helpers.go:52: drwxr-xr-x 4096 /tmp/snapshot-suite-Aufs-932587849/root/snapshots/1/fs
helpers.go:52: drwx------ 4096 /tmp/snapshot-suite-Aufs-932587849/root/snapshots/1/fs/dir1
helpers.go:50: -rw-r--r-- 5 /tmp/snapshot-suite-Aufs-932587849/root/snapshots/1/fs/dir1/f1 [ "Hello" ...]
helpers.go:52: drwx------ 4096 /tmp/snapshot-suite-Aufs-932587849/root/snapshots/1/fs/dir2
helpers.go:50: -rw-r--r-- 3 /tmp/snapshot-suite-Aufs-932587849/root/snapshots/1/fs/dir2/f2 [ "..." ...]
helpers.go:52: drwx------ 4096 /tmp/snapshot-suite-Aufs-932587849/root/snapshots/3
helpers.go:52: drwxr-xr-x 4096 /tmp/snapshot-suite-Aufs-932587849/root/snapshots/3/fs
helpers.go:52: drwxr-xr-x 4096 /tmp/snapshot-suite-Aufs-932587849/work
```

Start with testsuite.go:804-805 and the MoveFileFromLowerLayer test, then trace snapshot creation and mounting through testsuite/helpers.go:24, 46, 107, and 109. Reproduce the Travis failure if possible and determine why the test reports an invalid mount argument; done means the test passes reliably without masking snapshot errors.

Written by the indexing model from the issue text.