Assertions( assertJ 라이브러리 )

1. assertThat : 값 검증에 쓰임

assertThat(실제값). isEqualTo(기댓값)

assertThat(member.getName()).isEqualTo("hi3");

assertThat(실제 객체). isInstanceOf(객체 예상 타입)
assertThat(실제값). isNull()
등등 

2. asserThatThrownBy : 예외 발생 검증에 쓰임

asserThatThrownBy(() -> 예외를 발생시킬 로직). isInstanceOf(예외 클래스)

assertThatThrownBy(() -> memberService.createMember("hi1", 10))
			.isInstanceOf(IllegalStateException.class);

예외가 발생한다면 테스트를 통과, 발생하지 않는다면 실패