Add testcase definitions

[madmaxoft]

The checker will need more information on how to test the plugin as much as possible. While the special API implementations can add callback requests (such as when registering a command, we can add a request to call the handler with various params), the plugins will need more specific testing, such as specific commands in specific sequences. Or perhaps generating in-game events in a specific order (player connected, player executing a command, player interacting with the blocks, ...)

[madmaxoft]

This is now called "Scenario".

The Checker will receive a scenario file via commandline. It will load the scenario file and execute actions found within it. The scenario file itself will specify when exactly the plugin is to be loaded and initialized (plugin initialization may require setup, such as filesystem redirections).