test: add test-child-process-emfile fail message

When the test fails (as it does frequently on FreeBSD unfortunately)
provide a non-cryptic error message that also provides a line number.

PR-URL: #3335
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

test/sequential/test-child-process-emfile.js

@@ -27,8 +27,10 @@ proc.on('error', common.mustCall(function(err) {
   assert(err.code === 'EMFILE' || err.code === 'ENFILE');
 }));
 
-// 'exit' should not be emitted, the process was never spawned.
-proc.on('exit', assert.fail);
+proc.on('exit', function() {
+  const msg = '"exit" should not be emitted (the process never spawned!)';
+  assert.fail(null, null, msg);
+});
 
 // close one fd for LSan
 if (openFds.length >= 1) {