fixup! src: implement --trace-promises

Author: joyeecheung

test/parallel/test-process-env-allowed-flags-are-documented.js

@@ -106,6 +106,8 @@ assert(undocumented.delete('--no-node-snapshot'));
 assert(undocumented.delete('--loader'));
 assert(undocumented.delete('--verify-base-objects'));
 assert(undocumented.delete('--no-verify-base-objects'));
+assert(undocumented.delete('--trace-promises'));
+assert(undocumented.delete('--no-trace-promises'));
 
 // Remove negated versions of the flags.
 for (const flag of undocumented) {