add testbed for WP 5.9 and PHP 8.0, update actions

composer.json

@@ -24,7 +24,7 @@
     "dealerdirect/phpcodesniffer-composer-installer": "^0.7",
     "matthiasmullie/minify": "^1.3",
     "phpcompatibility/phpcompatibility-wp": "^2.1",
-    "phpunit/phpunit": "^5|^6|^7",
+    "phpunit/phpunit": "^5|^6|^7|^8",
     "slowprog/composer-copy-file": "~0.3",
     "squizlabs/php_codesniffer": "^3.6",
     "wp-coding-standards/wpcs": "^2.3",
@@ -65,5 +65,11 @@
       "minifyjs scripts/block.js > scripts/block.min.js",
       "minifyjs scripts/liveticker.js > scripts/liveticker.min.js"
     ]
+  },
+  "config": {
+    "allow-plugins": {
+      "composer/installers": true,
+      "dealerdirect/phpcodesniffer-composer-installer": true
+    }
   }
 }