Fix flakiness in BuildTriggerTest (#226)

There was the possibility of a race condition where the queue was empty
but the downstream job not yet added to it, resulting in assertions being
check too early and fail.

Show in a CI run under heavy load, not something usual, but this change
should remove the possibility

pom.xml

@@ -145,6 +145,18 @@
       <artifactId>token-macro</artifactId>
       <scope>test</scope>
     </dependency>
+    <dependency>
+      <groupId>org.awaitility</groupId>
+      <artifactId>awaitility</artifactId>
+      <version>4.1.1</version>
+      <scope>test</scope>
+      <exclusions>
+        <exclusion>
+          <groupId>org.hamcrest</groupId>
+          <artifactId>hamcrest</artifactId>
+        </exclusion>
+      </exclusions>
+    </dependency>
   </dependencies>
 
   <dependencyManagement>

src/test/java/hudson/plugins/parameterizedtrigger/test/BuildTriggerTest.java

@@ -30,7 +30,11 @@
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
+
+import static java.util.concurrent.TimeUnit.SECONDS;
 import static org.junit.Assert.*;
+
+import org.awaitility.Awaitility;
 import org.junit.Rule;
 import org.junit.Test;
 import hudson.tasks.Builder;
@@ -51,7 +55,7 @@ public class BuildTriggerTest {
     public LoggerRule logging = new LoggerRule().record(Run.class, Level.FINE);
 
     @Test
-    public void testParentProjectTrigger() throws Exception{
+    public void testParentProjectTrigger() throws Exception {
         FreeStyleProject downstream = r.createFreeStyleProject("downstream");
         MatrixProject upstream = r.createProject(MatrixProject.class, "upstream");
         List<ParameterDefinition> definition = new ArrayList<ParameterDefinition>();
@@ -64,16 +68,16 @@ public void testParentProjectTrigger() throws Exception{
         params.add(new CurrentBuildParameters());
         BuildTrigger triggerBuilder = new BuildTrigger(new BuildTriggerConfig("downstream", ResultCondition.SUCCESS, false, null, params, false));
         upstream.getPublishersList().add(triggerBuilder);
+
         r.buildAndAssertSuccess(upstream);
-        r.waitUntilNoActivity();
-        FreeStyleBuild downstreamBuild = downstream.getLastBuild();
-        assertNotNull("Downstream job should be triggered", downstreamBuild);
-        String project = downstreamBuild.getCause(Cause.UpstreamCause.class).getUpstreamProject();
+        Awaitility.await().pollInterval(1, SECONDS).atMost(10, SECONDS).until(() -> downstream.getLastBuild() != null);
+
+        String project = downstream.getLastBuild().getCause(Cause.UpstreamCause.class).getUpstreamProject();
         assertEquals("Build should be triggered by matrix project.", "upstream", project);
     }
 
     @Test
-    public void testChildProjectsTrigger() throws Exception{
+    public void testChildProjectsTrigger() throws Exception {
         MatrixProject upstream = r.createProject(MatrixProject.class, "upstream");
         FreeStyleProject downstream = r.createFreeStyleProject("downstream");