test: introduce common.crashOnUnhandledRejection

Ref: nodejs#12442
PR-URL: nodejs#12489
Backport-PR-URL: nodejs#13103
Reviewed-By: Matthew Loring <mattloring@google.com>
Reviewed-By: Julien Gilli <jgilli@nodejs.org>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: addaleax

test/common.js

@@ -552,3 +552,9 @@ exports.expectWarning = function(name, expected) {
     expected.splice(expected.indexOf(warning.message), 1);
   }, expected.length));
 };
+
+// Crash the process on unhandled rejections.
+exports.crashOnUnhandledRejection = function() {
+  process.on('unhandledRejection',
+             (err) => process.nextTick(() => { throw err; }));
+};