feat(sorting): add comparator for student group

app/src/test/java/sorting/StudentGroupComparator.java

@@ -0,0 +1,11 @@
+package sorting;
+
+import java.util.Comparator;
+
+public class StudentGroupComparator implements Comparator<Student> {
+
+    @Override
+    public int compare(Student s1, Student s2) {
+        return Integer.compare(s1.getGroupNumber(), s2.getGroupNumber());
+    }
+}