test: Category 도메인 테스트 검증 방식 변경

backend/ddang/src/test/java/com/ddang/ddang/category/domain/CategoryTest.java

@@ -4,6 +4,7 @@
 import org.junit.jupiter.api.DisplayNameGeneration;
 import org.junit.jupiter.api.DisplayNameGenerator;
 import org.junit.jupiter.api.Test;
+import org.springframework.test.util.ReflectionTestUtils;
 
 @DisplayNameGeneration(DisplayNameGenerator.ReplaceUnderscores.class)
 @SuppressWarnings("NonAsciiCharacters")
@@ -12,16 +13,20 @@ class CategoryTest {
     @Test
     void 카테고리_연관_관계를_세팅한다() {
         // given
-        Category main = new Category("main");
-        Category sub = new Category("sub");
+        final Category main = new Category("main");
+        final Category sub = new Category("sub");
+
+        ReflectionTestUtils.setField(main, "id", 1L);
+        ReflectionTestUtils.setField(sub, "id", 2L);
 
         // when
         main.addSubCategory(sub);
 
         // then
         SoftAssertions.assertSoftly(softAssertions -> {
             softAssertions.assertThat(main.getSubCategories()).hasSize(1);
-            softAssertions.assertThat(sub.getMainCategory()).isNotNull();
+            softAssertions.assertThat(main.getSubCategories()).contains(sub);
+            softAssertions.assertThat(sub.getMainCategory()).isEqualTo(main);
         });
     }
 }