Skip tests when curl is missing & suggest it in composer.

composer.json

@@ -38,6 +38,7 @@
     },
     "suggest": {
         "ext-openssl": "To use Security::encrypt() or have secure CSRF token generation.",
+        "ext-curl": "To enable more efficient network calls in Http\\Client.",
         "lib-ICU": "The intl PHP library, to use Text::transliterate() or Text::slug()"
     },
     "require-dev": {

tests/TestCase/Http/Client/Adapter/CurlTest.php

@@ -27,6 +27,8 @@ class CurlTest extends TestCase
     public function setUp()
     {
         parent::setUp();
+        $this->skipIf(!extension_exists('curl'), 'Skipping as ext/curl is not installed.');
+
         $this->curl = new Curl();
         $this->caFile = CORE_PATH . 'config' . DIRECTORY_SEPARATOR . 'cacert.pem';
     }