Add health check to phpunit webserver

[mvorisek]

Should fix random CI failures:

1) Atk4\Ui\Tests\DemosHttpNoExitTest::testDemoLateOutputErrorHeadersAlreadySent
GuzzleHttp\Exception\ConnectException: cURL error 7: Failed to connect to localhost port 9687 after 0 ms: Connection refused (see http://curl.haxx.se/libcurl/c/libcurl-errors.html)

/__w/ui/ui/vendor/guzzlehttp/guzzle/src/Handler/CurlFactory.php:186
/__w/ui/ui/vendor/guzzlehttp/guzzle/src/Handler/CurlFactory.php:150
/__w/ui/ui/vendor/guzzlehttp/guzzle/src/Handler/CurlFactory.php:103
/__w/ui/ui/vendor/guzzlehttp/guzzle/src/Handler/CurlHandler.php:43
/__w/ui/ui/vendor/guzzlehttp/guzzle/src/Handler/Proxy.php:28
/__w/ui/ui/vendor/guzzlehttp/guzzle/src/Handler/Proxy.php:51
/__w/ui/ui/vendor/guzzlehttp/guzzle/src/PrepareBodyMiddleware.php:37
/__w/ui/ui/vendor/guzzlehttp/guzzle/src/Middleware.php:30
/__w/ui/ui/vendor/guzzlehttp/guzzle/src/RedirectMiddleware.php:70
/__w/ui/ui/vendor/guzzlehttp/guzzle/src/Middleware.php:59
/__w/ui/ui/vendor/guzzlehttp/guzzle/src/HandlerStack.php:67
/__w/ui/ui/vendor/guzzlehttp/guzzle/src/Client.php:277
/__w/ui/ui/vendor/guzzlehttp/guzzle/src/Client.php:125
/__w/ui/ui/vendor/guzzlehttp/guzzle/src/Client.php:131
/__w/ui/ui/tests/DemosTest.php:217
/__w/ui/ui/tests/DemosTest.php:240
/__w/ui/ui/tests/DemosHttpTest.php:107
phpvfscomposer:///__w/ui/ui/vendor/phpunit/phpunit/phpunit:60