api_test: Fix PostContainersCreate

We can't look for the created file in the rwpath, because that
doesn't exist in the device-mapper world, instead look in the
RootfsPath.

api_test.go

@@ -646,13 +646,21 @@ func TestPostContainersCreate(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	if _, err := os.Stat(path.Join(container.rwPath(), "test")); err != nil {
+	if err := container.EnsureMounted(); err != nil {
+		t.Fatalf("Unable to mount container: %s", err)
+	}
+
+	if _, err := os.Stat(path.Join(container.RootfsPath(), "test")); err != nil {
 		if os.IsNotExist(err) {
 			utils.Debugf("Err: %s", err)
 			t.Fatalf("The test file has not been created")
 		}
 		t.Fatal(err)
 	}
+
+	if err := container.Unmount(); err != nil {
+		t.Fatalf("Unable to unmount container: %s", err)
+	}
 }
 
 func TestPostContainersKill(t *testing.T) {