Remove shelling out to mount

Also remove the target from the Mount struct because it should not be
used at all.  The target can be variable and set by a caller, not by the
snapshot drivers.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

snapshot/testutil/helpers.go

@@ -1,24 +1,13 @@
 package testutil
 
 import (
-	"os/exec"
+	"syscall"
 	"testing"
-
-	"github.com/docker/containerd"
 )
 
-func UnmountAll(t *testing.T, mounts []containerd.Mount) {
-	for _, mount := range mounts {
-		Unmount(t, mount.Target)
-	}
-}
-
 func Unmount(t *testing.T, mountPoint string) {
 	t.Log("unmount", mountPoint)
-	umount := exec.Command("umount", mountPoint)
-	err := umount.Run()
-	if err != nil {
-
+	if err := syscall.Unmount(mountPoint, 0); err != nil {
 		t.Error("Could not umount", mountPoint, err)
 	}
 }