Update dependency phpunit/phpunit to v8 [SECURITY]

Author: renovate[bot]

composer.json

@@ -22,7 +22,7 @@
     "php-coveralls/php-coveralls": "^2.2.0",
     "phpcompatibility/phpcompatibility-wp": "@stable",
     "phpunit/phpcov": "^5.0",
-    "phpunit/phpunit": "^7",
+    "phpunit/phpunit": "^8",
     "slowprog/composer-copy-file": "@stable",
     "wp-coding-standards/wpcs": "@stable",
     "xwp/wordpress-tests-installer": "@stable",