Add config validators

src/test/java/at/pcgamingfreaks/mkvaudiosubtitlechanger/util/TestUtil.java

@@ -0,0 +1,16 @@
+package at.pcgamingfreaks.mkvaudiosubtitlechanger.util;
+
+import at.pcgamingfreaks.mkvaudiosubtitlechanger.model.ConfigProperty;
+
+import java.util.Arrays;
+
+import static java.util.stream.Collectors.joining;
+
+public class TestUtil {
+    public static String yamlList(ConfigProperty main, ConfigProperty... child) {
+        return main.prop() + ":\n" + Arrays.stream(child)
+                .map(ConfigProperty::prop)
+                .collect(joining("\n", "  - ", ""));
+    }
+
+}