upgrade to phpunit 8+

[simevo]

I tried, it just requires this small fix:

--- a/tests/SpTest.php
+++ b/tests/SpTest.php
@@ -479,7 +479,7 @@ final class SpTest extends PHPUnit\Framework\TestCase
         }
     }
 
-    public static function tearDownAfterClass()
+    public static function tearDownAfterClass() : void
     {
         unlink(self::$settings['sp_key_file']);
         unlink(self::$settings['sp_cert_file']);

but I got many warnings similar to:

 ⚠ Can load from valid x m l
   │
   │ assertAttributeNotEmpty() is deprecated and will be removed in PHPUnit 9.
   │ readAttribute() is deprecated and will be removed in PHPUnit 9.
   │ getObjectAttribute() is deprecated and will be removed in PHPUnit 9.

these are related to: sebastianbergmann/phpunit#3338

rationale:

A test should not depend on private implementation details. It was a bad idea to make a bad testing practice this convenient.

upgrade path suggested by the creator of PHPUnit:

The upgrade path is to refactor your (test) code to not require the assertions in question.