Enable tests for PHP 8

Author: babenkoivan

.github/workflows/test.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-18.04
     strategy:
       matrix:
-        php: [ 7.1, 7.2, 7.3, 7.4 ]
+        php: [ 7.1, 7.2, 7.3, 7.4, 8.0 ]
     steps:
       - name: Checkout code
         uses: actions/checkout@v2