test: fix message test after V8 upgrade

With V8 6.2 there is one line less in the promise trace.

PR-URL: #15362
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

Author: targos

test/message/unhandled_promise_trace_warnings.out

@@ -21,7 +21,6 @@
     at getAsynchronousRejectionWarningObject (internal/process/promises.js:*)
     at rejectionHandled (internal/process/promises.js:*)
     at *
-    at Promise.then *
     at Promise.catch *
     at Immediate.setImmediate [as _onImmediate] (*test*message*unhandled_promise_trace_warnings.js:*)
     at *