test: ensure nextTick is not scheduled in exit

Previously our tests did not check this codepath as seen at
coverage.nodejs.org

PR-URL: https://github.com/nodejs/node/pull/9555
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

test/parallel/test-next-tick-when-exiting.js

@@ -0,0 +1,14 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+
+process.on('exit', () => {
+  assert.strictEqual(process._exiting, true, 'process._exiting was not set!');
+
+  process.nextTick(() => {
+    common.fail('process is exiting, should not be called.');
+  });
+});
+
+process.exit();