Run tests using headless chrome by default

Instead of opting out of headless mode (using `CI=true`), switch the jest-puppeteer configuration to use an opt-in environment variable (`HEADLESS=false`).

jest-puppeteer.config.js

 module.exports = {
   launch: {
-    headless: process.env.CI === 'true',
+    headless: process.env.HEADLESS !== 'false',
   },
   server: {
     command: 'npm run server -- -p 4488',