add a test for the free port finder

src/vs/base/test/node/port.test.ts

+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+'use strict';
+
+import * as assert from 'assert';
+import * as net from 'net';
+import ports = require('vs/base/node/ports');
+
+suite('Ports', () => {
+	test('Finds a free port', function (done: () => void) {
+
+		// get an initial freeport >= 7000
+		ports.findFreePort(7000, 100, (initialPort) => {
+			assert.ok(initialPort >= 7000);
+
+			// create a server to block this port
+			const server = net.createServer();
+			server.listen(initialPort, null, null, () => {
+
+				// once listening, find another free port and assert that the port is different from the opened one
+				ports.findFreePort(7000, 50, (freePort) => {
+					assert.ok(freePort >= 7000 && freePort !== initialPort);
+					server.close();
+
+					done();
+				});
+			});
+		});
+	});
+});
\ No newline at end of file