Flakes in WaitForBuildStepTest

Explicitly wait for the "wait" semaphore in downstream build, allowing the downstream build to go through full initialization.

Otherwise in some case you'd run into a timing issue where downstream build is not aborted as expected.

```
  10.218 [id=211]	WARNING	o.j.p.w.cps.CpsFlowExecution#saveOwner: Error persisting Run ds#1
java.nio.channels.ClosedByInterruptException
	at java.base/java.nio.channels.spi.AbstractInterruptibleChannel.end(AbstractInterruptibleChannel.java:199)
	at java.base/sun.nio.ch.FileChannelImpl.endBlocking(FileChannelImpl.java:171)
	at java.base/sun.nio.ch.FileChannelImpl.force(FileChannelImpl.java:472)
	at hudson.util.AtomicFileWriter.commit(AtomicFileWriter.java:234)
	at hudson.XmlFile.write(XmlFile.java:221)
	at org.jenkinsci.plugins.workflow.support.PipelineIOUtils.writeByXStream(PipelineIOUtils.java:30)
	at org.jenkinsci.plugins.workflow.job.WorkflowRun.save(WorkflowRun.java:1250)
	at org.jenkinsci.plugins.workflow.cps.CpsFlowExecution.saveOwner(CpsFlowExecution.java:2124)
	at org.jenkinsci.plugins.workflow.cps.CpsFlowExecution.start(CpsFlowExecution.java:622)
	at org.jenkinsci.plugins.workflow.job.WorkflowRun.run(WorkflowRun.java:335)
	at hudson.model.ResourceController.execute(ResourceController.java:101)
	at hudson.model.Executor.run(Executor.java:446)
 10.219 [us #1] [Pipeline] semaphore
  10.219 [us #1] [Pipeline] waitForBuild
  10.220 [us #1] Waiting for ds #1 to complete
  10.220 [ds #1] [Pipeline] Start of Pipeline
  10.241 [id=95]	INFO	o.j.p.w.t.s.SemaphoreStep$Execution#start: Blocking wait/1
  10.270 [ds #1] [Pipeline] semaphore
 180.002 [id=1]	WARNING	o.j.hudson.test.JenkinsRule$2#evaluate: Test timed out (after 180 seconds).
 [...]
```

Author: Vlatombe

src/test/java/org/jenkinsci/plugins/workflow/support/steps/build/WaitForBuildStepTest.java

@@ -173,6 +173,7 @@ public class WaitForBuildStepTest {
         SemaphoreStep.success("scheduled/1", true);
 
         WorkflowRun dsRun = ds.getBuildByNumber(1);
+        SemaphoreStep.waitForStart("wait/1", dsRun);
         waitForWaitForBuildAction(dsRun);
 
         // Abort the downstream build
@@ -201,6 +202,7 @@ public class WaitForBuildStepTest {
         SemaphoreStep.success("scheduled/1", true);
 
         WorkflowRun dsRun = ds.getBuildByNumber(1);
+        SemaphoreStep.waitForStart("wait/1", dsRun);
         waitForWaitForBuildAction(dsRun);
 
         // Abort the upstream build
@@ -229,6 +231,7 @@ public class WaitForBuildStepTest {
         SemaphoreStep.success("scheduled/1", true);
 
         WorkflowRun dsRun = ds.getBuildByNumber(1);
+        SemaphoreStep.waitForStart("wait/1", dsRun);
         waitForWaitForBuildAction(dsRun);
 
         // Abort the upstream build