test: fix test-process-exec-argv flakiness

Wait for the `close` event before parsing the child stdout output.

Fixes: https://github.com/nodejs/node/issues/6480
PR-URL: https://github.com/nodejs/node/pull/6575
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

test/parallel/test-process-exec-argv.js

@@ -15,7 +15,7 @@ if (process.argv[2] === 'child') {
     out += chunk;
   });
 
-  child.on('exit', function() {
+  child.on('close', function() {
     assert.deepStrictEqual(JSON.parse(out), execArgv);
   });
 }