test: fix flaky test-preload

Use `close` event rather than `exit` event to make sure all output has
been received before checking assertions.

PR-URL: #6728
Fixes: #6722
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

test/parallel/test-preload.js

@@ -81,7 +81,7 @@ var stdinStdout = '';
 stdinProc.stdout.on('data', function(d) {
   stdinStdout += d;
 });
-stdinProc.on('exit', function(code) {
+stdinProc.on('close', function(code) {
   assert.equal(code, 0);
   assert.equal(stdinStdout, 'A\nhello\n');
 });
@@ -97,7 +97,7 @@ var replStdout = '';
 replProc.stdout.on('data', function(d) {
   replStdout += d;
 });
-replProc.on('exit', function(code) {
+replProc.on('close', function(code) {
   assert.equal(code, 0);
   const output = [
     'A',