fix(测试跟踪): 关联需求检索后，文字太长会出现重叠

--bug=1037578 --user=陈建星 【测试跟踪】github#29619，功能用例编辑页面-相关需求搜索关键字后，显示出来的字体重叠 https://www.tapd.cn/55049933/s/1620835

test-track/frontend/src/business/case/components/case/CaseBaseInfo.vue

@@ -231,6 +231,7 @@
                     :filter-method="filterDemand"
                     @click.native="onClick"
                     size="small"
+                    popper-class="demand-search-suggestion"
                   >
                     <template slot-scope="{ data }">
                       <div class="story-box">
@@ -782,6 +783,13 @@ export default {
 };
 </script>
 
+<style lang="scss">
+.demand-search-suggestion .el-cascader__suggestion-item {
+  white-space: nowrap;
+  max-width: 400px;
+}
+</style>
+
 <style scoped lang="scss">
 @import "@/business/style/index.scss";
 .case-padding{