@Test
void 문자열로_직전월을_구한다() throws Exception {

    //given
    String month = "2020-09";
    YearMonthRequestDto dto = YearMonthRequestDto.builder()
            .strYearMonth(month)
            .build();
    //when
    String beforeMonth = dto.getBeforeMonthByString();
    //then
    assertThat(beforeMonth).isEqualTo("2020-08");
}