Fix unit tests failing in PHP 8 in CI

The problem was that PHP 8 requires setting up a different version of
PHPUnit, and after doing that two versions were in place, one in
'vendor/phpunit' and another one in 'bin/composer/phpunit'; the
autoloader was picking the former but it should pick the later.

.github/workflows/ci.yml

@@ -66,6 +66,8 @@ jobs:
              composer bin phpunit config --unset platform
              composer bin phpunit config repositories.0 '{"type": "path", "url": "/tmp/phpunit-7.5-fork/phpunit-add-compatibility-with-php8-to-phpunit-7", "options": {"symlink": false}}'
              composer bin phpunit require --dev -W phpunit/phpunit:@dev --ignore-platform-reqs
+             rm -rf ./vendor/phpunit/
+             composer dump-autoload
            fi
 
       - name: Init DB and WP

.github/workflows/pr-unit-tests.yml

@@ -63,6 +63,8 @@ jobs:
              composer bin phpunit config --unset platform
              composer bin phpunit config repositories.0 '{"type": "path", "url": "/tmp/phpunit-7.5-fork/phpunit-add-compatibility-with-php8-to-phpunit-7", "options": {"symlink": false}}'
              composer bin phpunit require --dev -W phpunit/phpunit:@dev --ignore-platform-reqs
+             rm -rf ./vendor/phpunit/
+             composer dump-autoload
            fi
 
       - name: Init DB and WP