Only activate TestExtension for blockSynchronously in the relevant te…

…st method

Co-authored-by: Jesse Glick <[email protected]>

src/test/java/org/jenkinsci/plugins/workflow/flow/FlowExecutionListTest.java

@@ -273,7 +273,7 @@ private enum State {
             UNBLOCKED,
         }
 
-        @TestExtension public static class DescriptorImpl extends StepDescriptor {
+        @TestExtension("stepExecutionIteratorDoesNotLeakBuildsWhenOneIsStuck") public static class DescriptorImpl extends StepDescriptor {
             @Override
             public Set<? extends Class<?>> getRequiredContext() {
                 return Collections.singleton(TaskListener.class);