Add sync before unmount on snapshotter layer test

Signed-off-by: Derek McGowan <derek@mcg.dev>

snapshots/testsuite/testsuite.go

@@ -959,6 +959,7 @@ func check128LayersMount(name string) func(ctx context.Context, t *testing.T, sn
 				t.Fatalf("[layer %d] preparing doesn't equal to flat after apply: %+v", i, err)
 			}
 
+			sync()
 			testutil.Unmount(t, preparing)
 
 			parent = filepath.Join(work, fmt.Sprintf("committed-%d", i))

snapshots/testsuite/testsuite_unix.go

@@ -18,7 +18,11 @@
 
 package testsuite
 
-import "syscall"
+import (
+	"syscall"
+
+	"golang.org/x/sys/unix"
+)
 
 func clearMask() func() {
 	oldumask := syscall.Umask(0)
@@ -26,3 +30,7 @@ func clearMask() func() {
 		syscall.Umask(oldumask)
 	}
 }
+
+func sync() {
+	unix.Sync()
+}

snapshots/testsuite/testsuite_windows.go

@@ -19,3 +19,5 @@ package testsuite
 func clearMask() func() {
 	return func() {}
 }
+
+func sync() {}