What is the best practice for testing multiple player features of a plugin when you're the only one on a server?

Thanks

I always find a good server owner and have them test fetures of plugins im working on.

Depends on what you are testing. Mostly I just create custom commands that allow me to alter the game world to essentially mock the circumstances I want to test.

If you want to test what happens for an object placed by a given player, simply create a command that assigns an arbitrary owner to that object. That allows testing things like Prevent Looting or No Escape.

If you need to test friends or clans integration, edit the data file to add somebody's steam ID to your clan or friends, then assign an object to them.

If need a player object, spawn a player. If you need that player to have state (health, hostile, equipment), simply supply that state in your plugin code. The Inventory Viewer plugin can allow easily giving then items. If you need the player to be mounted, create commands to can run to mount them to a specified seat.

You can also temporarily edit your plugin code to just hard code certain state to test the other player's side of the interaction. You can also make this an undocumented feature of the plugin that is designed for testing, in case you or another dev needs to work on the plugin in the future.