<?php
header("Link: <   //wut.com.test/>; rel=preconnect");
?>
<!DOCTYPE html>
<script>
    if (window.testRunner) {
        testRunner.dumpAsText();
        testRunner.waitUntilDone();
    }
    if (window.internals) {
        internals.settings.setLogDnsPrefetchAndPreconnect(true);
    }
    if (!localStorage.getItem("reloaded")) {
        localStorage.setItem("reloaded",  true);
        location.reload();
    } else {
        localStorage.removeItem("reloaded");
    }
</script>
This test check if a Link header triggered a dns prefetch.
<script>
    if (window.testRunner)
        testRunner.notifyDone();
</script>