Create variant for testing Redis-related plugins

plugins/templates/before-script-redis.yml

+before_script:
+  # Set up WordPress tests
+  - bash bin/install-wp-tests.sh $MYSQL_DATABASE root $MYSQL_ROOT_PASSWORD mysql $WP_VERSION true
+
+  # PHPUnit
+  - |
+    if [[ $(php -v) =~ "PHP 8." ]]; then
+      composer global require "phpunit/phpunit=9.*"
+    elif [[ $(php -v) =~ "PHP 7." ]]; then
+      composer global require "phpunit/phpunit=6.1.*"
+    elif [[ $(php -v) =~ "PHP 5.6" ]]; then
+      composer global require "phpunit/phpunit=5.7.*"
+    fi
+
+  # PHPUnit requirements
+  - composer global require yoast/phpunit-polyfills
+
+  # PECL Redis module
+  - pecl install redis
+  - echo "extension=redis.so" > /usr/local/etc/php/conf.d/redis.ini