Tests: fix incorrect class/file name for test class

The `src` class being tested is called `Cookie`, so the test class should be called `CookieTest`, not `CookiesTest`.

tests/CookiesTest.php → tests/CookieTest.php

@@ -14,7 +14,7 @@
 use WpOrg\Requests\Tests\TestCase;
 use WpOrg\Requests\Utility\CaseInsensitiveDictionary;
 
-final class CookiesTest extends TestCase {
+final class CookieTest extends TestCase {
 	public function testBasicCookie() {
 		$cookie = new Cookie('requests-testcookie', 'testvalue');