fix(测试跟踪): 导出时字段名称过长显示问题

--bug=1022756 --user=宋昌昌 【测试跟踪】导出功能用例，导出窗口中自定义字段名称过长会互相遮挡 https://www.tapd.cn/55049933/s/1336985

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

@@ -11,11 +11,9 @@
     <el-dropdown-menu>
       <el-dropdown-item :disabled="disable" command="STEP">
         <div>{{ $t("test_track.case.step_describe") }}</div>
-        <!--          <div>{{ $t('test_track.case.step_describe_tip') }}</div>-->
       </el-dropdown-item>
       <el-dropdown-item :disabled="disable" command="TEXT">
         <div>{{ $t("test_track.case.text_describe") }}</div>
-        <!--          <div>{{ $t('test_track.case.text_describe_tip') }}</div>-->
       </el-dropdown-item>
     </el-dropdown-menu>
   </el-dropdown>

test-track/frontend/src/business/case/components/export/TestCaseExportFieldList.vue

@@ -9,7 +9,7 @@
           v-model="item.enable"
           :disabled="item.disabled"
           @change="change">
-          {{ item.name }}
+          {{ item.name | ellipsis}}
         </el-checkbox>
       </el-col>
     </span>
@@ -38,6 +38,17 @@ export default {
     change(value) {
       this.$emit('enableChange', value);
     }
+  },
+  filters: {
+    ellipsis(value) {
+      if (!value) {
+        return '';
+      }
+      if (value.length > 5) {
+        return value.slice(0, 4) + '...';
+      }
+      return value;
+    },
   }
 }
 </script>