then:
        1 * reportRenderer.render(_, _) >> { String model, HtmlReportBuilder builder ->
            builder.renderHtmlPage("index.html", model, pageRenderer)
        }
        1 * pageRenderer.render(_, _) >> { String model, HtmlPageBuilder<SimpleHtmlWriter> builder ->
            builder.output.startElement("pre").characters(model).endElement()
        }

        and:

Class <org.gradle.api.internal.tasks.testing.report.PackageTestResults> is not annotated (directly or via its package) with @org.gradle.api.NonNullApi in (PackageTestResults.java:0)
Class <org.gradle.api.internal.tasks.testing.report.PageRenderer$1> is not annotated (directly or via its package) with @org.gradle.api.NonNullApi in (PageRenderer.java:0)