Fixing flaky scheduler test (#1411)

There was an implicit race condition between the start().join() and the
scheduledInstances()

Signed-off-by: fjtirado <ftirados@redhat.com>

Author: fjtirado

impl/test/src/test/java/io/serverlessworkflow/impl/test/SchedulerTest.java

@@ -21,6 +21,7 @@
 
 import io.serverlessworkflow.impl.WorkflowApplication;
 import io.serverlessworkflow.impl.WorkflowDefinition;
+import io.serverlessworkflow.impl.WorkflowInstance;
 import java.io.IOException;
 import java.time.Duration;
 import java.util.Map;
@@ -48,8 +49,9 @@ static void tearDown() throws IOException {
   void testAfter() throws IOException, InterruptedException, ExecutionException {
     try (WorkflowDefinition def =
         appl.workflowDefinition(readWorkflowFromClasspath("workflows-samples/after-start.yaml"))) {
-      def.instance(Map.of()).start().join();
+      WorkflowInstance instance = def.instance(Map.of());
       assertThat(def.scheduledInstances()).isEmpty();
+      instance.start().join();
       await()
           .pollDelay(Duration.ofMillis(50))
           .atMost(Duration.ofMillis(200))