Add PHP 8 to test matrix

Author: shengslogar

.github/workflows/tests.yml

@@ -13,7 +13,7 @@ jobs:
     phpunit:
         strategy:
             matrix:
-                php: [7.4, 7.3, 7.2]
+                php: [8.0, 7.4, 7.3, 7.2]
                 laravel: [8.*, 7.*, 6.*]
                 os: [ubuntu-latest, windows-latest]