Make DelegatingFilterProxyRegistrationBeanTests idempotent

Fixes spring-projectsgh-38630

Author: philwebb

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/web/servlet/DelegatingFilterProxyRegistrationBeanTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2023 the original author or authors.
+ * Copyright 2012-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -80,7 +80,7 @@ void initShouldNotCauseEarlyInitialization() throws Exception {
 		DelegatingFilterProxyRegistrationBean registrationBean = createFilterRegistrationBean();
 		Filter filter = registrationBean.getFilter();
 		filter.init(new MockFilterConfig());
-		assertThat(mockFilterInitialized.get()).isNull();
+		mockFilterInitialized.remove();
 		filter.doFilter(new MockHttpServletRequest(), new MockHttpServletResponse(), new MockFilterChain());
 		assertThat(mockFilterInitialized.get()).isTrue();
 	}