Flaky test: MoveFileFromLowerLayer

[AkihiroSuda]

Observed in https://travis-ci.org/containerd/aufs/builds/309042106 (merge commit for #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