CpsFlowExecutionTest.iterateAfterSuspend flaky

[jglick]

Introduced in #797, and seems to flake a lot, e.g.: https://github.com/jenkinsci/workflow-cps-plugin/runs/31557729869

Reproducible using

diff --git plugin/src/main/java/org/jenkinsci/plugins/workflow/cps/CpsThreadGroup.java plugin/src/main/java/org/jenkinsci/plugins/workflow/cps/CpsThreadGroup.java
index 51518e6f..519e1f14 100644
--- plugin/src/main/java/org/jenkinsci/plugins/workflow/cps/CpsThreadGroup.java
+++ plugin/src/main/java/org/jenkinsci/plugins/workflow/cps/CpsThreadGroup.java
@@ -652,6 +652,11 @@ public final class CpsThreadGroup implements Serializable {
     }
 
     void shutdown() {
+        try {
+            Thread.sleep(1000);
+        } catch (InterruptedException x) {
+            x.printStackTrace();
+        }
         runner.shutdown();
     }