runtime: Move mockfs control global into mockfs.go

virtcontainers/persist/fs/mockfs.go defines a mock filesystem type for
testing.  A global variable in virtcontainers/persist/manager.go is used to
force use of the mock fs rather than a normal one.

This patch moves the global, and the EnableMockTesting() function which
sets it into mockfs.go.  This is slightly cleaner to begin with, and will
allow some further enhancements.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

src/runtime/virtcontainers/virtcontainers_test.go

@@ -15,7 +15,6 @@ import (
 	"syscall"
 	"testing"
 
-	"github.com/kata-containers/kata-containers/src/runtime/virtcontainers/persist"
 	"github.com/kata-containers/kata-containers/src/runtime/virtcontainers/persist/fs"
 	"github.com/kata-containers/kata-containers/src/runtime/virtcontainers/utils"
 	"github.com/sirupsen/logrus"
@@ -108,7 +107,7 @@ func setupClh() {
 func TestMain(m *testing.M) {
 	var err error
 
-	persist.EnableMockTesting()
+	fs.EnableMockTesting()
 
 	flag.Parse()