osbuild2: add missing JSON unmarshaling test for org.osbuild.dracut stage

The `org.osbuild.dracut` stage was previously not tested for JSON
unmarshalling. Add the missing test case and extend Stage's
`UnmarshalJSON()` method.

Signed-off-by: Tomas Hozza <thozza@redhat.com>

internal/osbuild2/stage.go

@@ -87,6 +87,8 @@ func (stage *Stage) UnmarshalJSON(data []byte) error {
 		options = new(TimezoneStageOptions)
 	case "org.osbuild.chrony":
 		options = new(ChronyStageOptions)
+	case "org.osbuild.dracut":
+		options = new(DracutStageOptions)
 	case "org.osbuild.dracut.conf":
 		options = new(DracutConfStageOptions)
 	case "org.osbuild.keymap":

internal/osbuild2/stage_test.go

@@ -62,6 +62,16 @@ func TestStage_UnmarshalJSON(t *testing.T) {
 				data: []byte(`{"type":"org.osbuild.chrony","options":{"timeservers":null}}`),
 			},
 		},
+		{
+			name: "dracut",
+			fields: fields{
+				Type:    "org.osbuild.dracut",
+				Options: &DracutStageOptions{},
+			},
+			args: args{
+				data: []byte(`{"type":"org.osbuild.dracut","options":{"kernel":null}}`),
+			},
+		},
 		{
 			name: "dracut.conf",
 			fields: fields{