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:

• 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
• QR codes for enabled and disabled short URLs
• ...

[acelaya]

Depends on #1786