Move WithMounts to specs

Signed-off-by: Maksym Pavlenko <pavlenko.maksym@gmail.com>

pkg/os/os_unix.go

@@ -21,18 +21,8 @@ package os
 import (
 	"os"
 	"path/filepath"
-
-	"github.com/containerd/containerd/mount"
 )
 
-// UNIX collects unix system level operations that need to be
-// mocked out during tests.
-type UNIX interface {
-	Mount(source string, target string, fstype string, flags uintptr, data string) error
-	Unmount(target string) error
-	LookupMount(path string) (mount.Info, error)
-}
-
 // ResolveSymbolicLink will follow any symbolic links
 func (RealOS) ResolveSymbolicLink(path string) (string, error) {
 	info, err := os.Lstat(path)