CI: Basic Test Shop Installation

[HMKnapp]

On every pushed commit, CI

• creates temporary external hostname with NGROK
• bootstraps a Wordpress installation via wp-cli from composer
• installs WooCommerce plugin
• fills shop with sample shop items
• packages QENTA Checkout Seamless plugin from current commit and installs it
• runs (dummy) test with curl over ngrok

To add: E2E testing of shopping and payment processes