fix(性能测试): 修复性能测试修改单位后不能调整预热时间的问题

--bug=1008711 --user=刘瑞斌
[github#8398]性能测试并发1000，压测时长1小时时，在多少秒内增加用户无法修改，恒为1
https://www.tapd.cn/55049933/s/1079454

Closes #8398

frontend/src/business/components/performance/test/components/PerformancePressureConfig.vue

@@ -453,8 +453,8 @@ export default {
           tg.threadType === 'ITERATION') {
           continue;
         }
-        if (tg.duration < tg.rampUpTime) {
-          tg.rampUpTime = tg.duration;
+        if (this.getDuration(tg) < tg.rampUpTime) {
+          tg.rampUpTime = this.getDuration(tg);
         }
         if (tg.rampUpTime < tg.step) {
           tg.step = tg.rampUpTime;