test(VolunteerQueryService): deleteAllInBatch 추가

- getRankingByHours_noVolunteers 오류 해결

Author: m-a-king

src/test/java/com/somemore/volunteer/service/VolunteerQueryServiceTest.java

@@ -181,6 +181,9 @@ void getRankingByHours() {
     @DisplayName("등록된 봉사자가 없는 경우 빈 랭킹 리스트를 반환한다.")
     @Test
     void getRankingByHours_noVolunteers() {
+        // given
+        volunteerRepository.deleteAllInBatch();
+
         // when
         VolunteerRankingResponseDto response = volunteerQueryService.getRankingByHours();