adam bien's blog

import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.Produces;
import javax.ws.rs.core.MediaType;
import javax.ws.rs.core.Response;

import org.eclipse.microprofile.rest.client.inject.RegisterRestClient;

@Path("/SHOULD_NOT_EXIST")
@RegisterRestClient(baseUri = "https://microprofile.io/")
public interface ProblematicResourceClient {

    @GET
    @Produces(MediaType.TEXT_PLAIN)
    Response fetchNotExistingContent();
}

will cause an WebApplicationException like e.g.:

javax.ws.rs.WebApplicationException: Unknown error, status code 404
at org.jboss.resteasy.microprofile.client.DefaultResponseExceptionMapper.toThrowable(DefaultResponseExceptionMapper.java:21)
(...)   

To test the 404 status code:

import io.quarkus.test.junit.QuarkusTest;
import static org.junit.jupiter.api.Assertions.assertEquals;
import javax.inject.Inject;
import org.eclipse.microprofile.rest.client.inject.RestClient;
import org.junit.jupiter.api.Test;

@QuarkusTest
public class ProblematicResourceClientIT {

    @Inject
    @RestClient
    ProblematicResourceClient cut;

    @Test
    public void callNotExistingResource() {
        var response = this.cut.fetchNotExistingContent();
        assertEquals(response.getStatus(), 404);
    }
}

...you need to set the MP config parameter to true::

microprofile.rest.client.disable.default.mapper=true

Now the Response contains the status code 404 and the test passes.