test: setup sandbox on linux (#3530)

Drop all the `--no-sandbox` bits from tests and infrastructure. Instead, configure
Travis to enable user namespace clone.

test/test.js

@@ -47,7 +47,6 @@ const defaultBrowserOptions = {
   slowMo,
   headless,
   dumpio: (process.env.DUMPIO || 'false').trim().toLowerCase() === 'true',
-  args: ['--no-sandbox']
 };
 
 let parallel = 1;