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

See gh-47263

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

@@ -29,3 +29,7 @@ dependencies {
 	testImplementation(project(":starter:spring-boot-starter-test"))
 	testImplementation("io.projectreactor:reactor-test")
 }
+
+tasks.named("compileTestJava") {
+	options.nullability.checking = "tests"
+}