@Test
    void tlsCustomizerWithoutTlsSelfSigned() {
        // Called `tlsSelfSigned()` for default host but did not for virtual host.
        assertThatThrownBy(() -> Server.builder()
                                       .tlsSelfSigned()
                                       .virtualHost("example.com")
                                       .tlsCustomizer(unused -> {})
                                       .and().build())
                .isInstanceOf(IllegalStateException.class)
                .hasMessageContaining("tlsCustomizer");
    }
Makes sure that {@link ServerBuilder#decorator(DecoratingHttpServiceFunction)} works at every service and
virtual hosts and {@link VirtualHostBuilder#decorator(DecoratingHttpServiceFunction)} works only at
its own services.
core/src/test/java/com/linecorp/armeria/server/ServerBuilderTest.java