add thens to catch errors

lib/tests/run_tests.js

@@ -137,9 +137,10 @@ module.exports = {
             }
             console.log(`Coverage report created. You can find it here: ${fs.dappPath('coverage/index.html')}\n`);
             const opn = require('opn');
-            opn(fs.dappPath('coverage/index.html'));
-
-            setTimeout(next, 1000); // Slight delay to enable the browser to open. opn's promise calls too fast
+            const _next = () => { next(); };
+            opn(fs.dappPath('coverage/index.html'), {wait: false})
+              .then(() => new Promise(resolve => setTimeout(resolve, 1000)))
+              .then(_next, _next);
           });
       }
     ], (err) => {