Add tests

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

@@ -0,0 +1,20 @@
+package at.pcgamingfreaks.mkvaudiosubtitlechanger.util;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+import java.util.Set;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class SetUtilsTest {
+
+    @Test
+    void retainOf() {
+        List<Integer> list1 = List.of(1, 2, 3, 4, 5);
+        List<Integer> list2 = List.of(2, 4, 6, 8, 10);
+        Set<Integer> expected = Set.of(2, 4);
+
+        assertEquals(expected, SetUtils.retainOf(list1, list2));
+    }
+}