test: fix ClientTest relative URI rejected by Guzzle 7.x stricter validation

Newer Guzzle 7.x patch versions reject relative URIs (e.g. '/') when no
base_uri is configured, throwing InvalidArgumentException before the
MockHandler is reached. Replace with an absolute URI so the mock handler
intercepts correctly on all Guzzle 7.x versions.

Author: mehmet-yoti

tests/Http/ClientTest.php

@@ -37,7 +37,7 @@ public function testSendRequest()
 
         $this->assertSame(
             $someResponse,
-            $client->sendRequest(new Request('GET', '/'))
+            $client->sendRequest(new Request('GET', 'https://api.yoti.com/'))
         );
 
         $this->assertEquals(30, $someHandler->getLastOptions()['timeout']);
@@ -138,6 +138,6 @@ private function sendRequestAndThrow(\Throwable $exception)
 
         $client = new Client(['handler' => $someHandlerStack]);
 
-        $client->sendRequest(new Request('GET', '/'));
+        $client->sendRequest(new Request('GET', 'https://api.yoti.com/'));
     }
 }