Mock out the render method on a tagLib so that it's easy to verify the template and model utilized by the tagLib when rendering

TagLibRenderVerification

// Setup
    private mockTagLibRender() {
        tagLib.metaClass.render = { Map attrs ->
            render = attrs
        }
    }
    
// Utilization
        given:
        mockTagLibRender()
        CoolThing coolThing = new CoolThing()

        when:
        tagLib.renderCoolThing(
                now: "now",
                derp: "derp"
        )

        then:
        render.template == "this/template/is/used/to/render"
        render.model.now == "now"
        render.model.derp == "derp"