Switch to headless browsers on the bots

The UI is of no use on the bots since we can only see the console logs,
so not having to start it might be faster and less error-prone.

test/test.js

@@ -763,7 +763,7 @@ function unitTestPostHandler(req, res) {
   return true;
 }
 
-async function startBrowser(browserName, startUrl) {
+async function startBrowser(browserName, startUrl, headless = false) {
   const revisions = require("puppeteer/package.json").puppeteer;
   const wantedRevision =
     browserName === "chrome"
@@ -788,7 +788,7 @@ async function startBrowser(browserName, startUrl) {
 
   const browser = await puppeteer.launch({
     product: browserName,
-    headless: false,
+    headless,
     defaultViewport: null,
     // Firefox must complete its execution before starting, mainly on Windows.
     // Refer to https://github.com/puppeteer/puppeteer/issues/5376 and
@@ -828,8 +828,9 @@ function startBrowsers(rootUrl, initSessionCallback) {
       `&delay=${options.statsDelay}` +
       `&masterMode=${options.masterMode}`;
     const startUrl = rootUrl + queryParameters;
+    const headless = options.strictVerify;
 
-    startBrowser(browserName, startUrl)
+    startBrowser(browserName, startUrl, headless)
       .then(function (browser) {
         session.browser = browser;
         if (initSessionCallback) {