test: [#6] ReservationDateTest 생성

1. createReservationDateWhichIsNotBetweenOneAndThirtyOne()
- validateReservationDate()
 1 ~ 31일 사이의 날짜가 아니면 IllegalArgumentException 발생시키는지 테스트

src/test/java/christmas/ReservationDateTest.java

@@ -0,0 +1,19 @@
+package christmas;
+
+import christmas.domain.ReservationDate;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.AssertionsForClassTypes.assertThatThrownBy;
+
+public class ReservationDateTest {
+    @DisplayName("예약 날짜가 12월(1~31일)에 해당하지 않으면 예외발생")
+    @Test
+    void createReservationDateWhichIsNotBetweenOneAndThirtyOne() {
+        assertThatThrownBy(() -> new ReservationDate(0))
+                .isInstanceOf(IllegalArgumentException.class);
+
+        assertThatThrownBy(() -> new ReservationDate(32))
+                .isInstanceOf(IllegalArgumentException.class);
+    }
+}