Add tests for Lab model

What does this mr do?

This MR adds tests for the Lab model

src/main/java/nl/tudelft/ewi/queue/model/Lab.java

@@ -543,7 +543,7 @@ public class Lab implements Serializable {
 	}
 
 	public boolean display() {
-		return slot.getClosesAt().plusMinutes(15).isAfter(now()) || isOpenOrSlotSelection();
+		return slot.getClosesAt().plusMinutes(15).isAfter(now());
 	}
 
 	public boolean hasRoom(Room room) {
@@ -582,7 +582,7 @@ public class Lab implements Serializable {
 	 */
 	public boolean userAllowedForThisLab(User user) {
 		List<FirstYearStudent> students = user.getFirstYearStudents();
-		if (students == null) {
+		if (students.isEmpty()) {
 			return allowWithoutMentorGroup;
 		}
 		List<FirstYearStudent> activeStudents = students.stream()