Support running API tests with certain server configuration

[acelaya]

API tests depend on the global app config to pass, which means it's also not possible to test different config combinations.

Add this, so that a test can load some new config, restart the server, and run the tests against that config.

Scenarios to test:

• [x] Error returned by /mercure-info endpoint when mercure is not configured
• [ ] "Not found" redirects, both global and domain-specific
• [ ] Redis integration (single instance, cluster and sentinels)
• [ ] Short URL deletion when visits threshold is reached
• [ ] Support for multi-segment slugs
• [ ] Support for trailing slashes in short URLs
• [ ] Short URLs with and without trailing slash, combined with multi-segment slugs
• [ ] Multiple status codes for short URLs
• [ ] strict and loose mode
• [ ] Redirect extra path mode (append, ignore)
• [x] CORS options (allow origin, max age and allow credentials)
• [ ] Visitor IP resolution with TRUSTED_PROXIES.
• [ ] ...

[acelaya]

Depends on https://github.com/shlinkio/shlink/issues/1786