upgrade to phpunit 8+

I tried, it just requires this small fix:
```diff
--- 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: https://github.com/sebastianbergmann/phpunit/issues/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.

Start with tests/SpTest.php and the deprecated assertions reported when running the test suite under PHPUnit 8+. Review the PHPUnit 3338 upgrade guidance and identify each use of private-attribute assertions or attribute access. Done means the tests run on PHPUnit 8+ without the reported deprecation warnings.

Written by the indexing model from the issue text.