Add missing @Test annotation

See gh-17012

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/embedded/UndertowWebServerFactoryCustomizerTests.java

@@ -100,6 +100,7 @@ public class UndertowWebServerFactoryCustomizerTests {
 		assertThat(boundServerOption(UndertowOptions.MAX_HEADER_SIZE)).isNull();
 	}
 
+	@Test
 	public void customMaxHttpHeaderSizeIgnoredIfZero() {
 		bind("server.max-http-header-size=0");
 		assertThat(boundServerOption(UndertowOptions.MAX_HEADER_SIZE)).isNull();