Add multi-plugin testing

[madmaxoft]

Using scenario files' initializePlugin action it would be possible to load multiple plugins and test them together (allowing testing the inter-plugin calls).

However, it should still be possible to use the Checker in the classic single-plugin way, using the -p parameter.

[madmaxoft]

Note: The simulator currently has a single sandbox for the plugin; this will need extending, so that separate plugins don't overwrite each other's globals.

[NiLSPACE]

Can't we create a new Simulator for each plugin? Also, we could even make it so cPluginManager:CallPlugin works :)

[madmaxoft]

The Simulator contains everything - hook registrations, command handlers, worlds, players etc., so it'd be easier to have multiple sandboxes in a single Simulator. And yes, making cPluginManager:CallPlugin() work is the main intent of this issue.

I see it as low priority, though. First we want to be able to test plugins as best as we can.