Add nullability annotations to tests in smoke-test/spring-boot-smoke-test-servlet

See gh-47263

smoke-test/spring-boot-smoke-test-servlet/build.gradle

@@ -30,3 +30,7 @@ dependencies {
 
 	testRuntimeOnly(project(":starter:spring-boot-starter-tomcat"))
 }
+
+tasks.named("compileTestJava") {
+	options.nullability.checking = "tests"
+}