test: favor assertions over console logging

Communicate about leaked globals via `AssertionError` rather than
`console.log()`.

PR-URL: nodejs#11547
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/common.js

@@ -399,8 +399,7 @@ process.on('exit', function() {
   if (!exports.globalCheck) return;
   const leaked = leakedGlobals();
   if (leaked.length > 0) {
-    console.error('Unknown globals: %s', leaked);
-    fail('Unknown global found');
+    fail(`Unexpected global(s) found: ${leaked.join(', ')}`);
   }
 });