test: add test for debug usage message

PR-URL: https://github.com/nodejs/node/pull/8061
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: cjihrig - Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-debug-usage.js

@@ -0,0 +1,21 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const spawn = require('child_process').spawn;
+
+const child = spawn(process.execPath, ['debug']);
+child.stderr.setEncoding('utf8');
+
+const expectedUsageMessage = `Usage: node debug script.js
+       node debug <host>:<port>
+       node debug -p <pid>
+`;
+var actualUsageMessage = '';
+child.stderr.on('data', function(data) {
+  actualUsageMessage += data.toString();
+});
+
+child.on('exit', common.mustCall(function(code) {
+  assert.strictEqual(code, 1);
+  assert.strictEqual(actualUsageMessage, expectedUsageMessage);
+}));