Use current time as start time for OMTG

When we use the test start time via JMeterContextService#getTestStartTime,
we might create a storm of test samples at the beginning, when a setup thread group
made us wait.

Author: FSchumacher

src/core/src/main/kotlin/org/apache/jmeter/threads/openmodel/OpenModelThreadGroup.kt

@@ -204,7 +204,7 @@ public class OpenModelThreadGroup :
             val seed = randomSeed
             val rnd = if (seed == 0L) Random() else Random(seed)
             val gen = ThreadScheduleProcessGenerator(rnd, parsedSchedule)
-            val testStartTime = JMeterContextService.getTestStartTime()
+            val testStartTime = System.currentTimeMillis()
             val executorService = Executors.newCachedThreadPool()
             this.executorService = executorService
             val starter = ThreadsStarter(testStartTime, executorService, activeThreads, gen) { threadNumber ->