@Test
    public void addsExplicitValidators() {
        ValidationEvent event = ValidationEvent.builder()
                .severity(Severity.ERROR)
                .id("Foo")
                .message("bar")
                .build();
        String document = "{\"smithy\": \"1.0\"}";
        ValidatedResult<Model> result = new ModelAssembler()
                .addUnparsedModel("[N/A]", document)
                .addValidator(index -> Collections.singletonList(event))
                .assemble();

        assertThat(result.getValidationEvents(), contains(event));
    }
This test exercises ModelValidator but done so using the ModelAssembler.
smithy-model/src/test/java/software/amazon/smithy/model/loader/ModelValidatorTest.java