Add test environment

[snipercup]

As requested by Khaligufzel, a way to quickly test a map in-game.

Adds a save & test button to the map editor that will send you to the test environment and load the map you were editing. This testenvironment has the bare minimum. Some lights, the chunk representing the map and player movement. There is no inventory, overmap or any UI except for a menu to return to the content editor. You will have to open the map again to continue editing.

It may be improved in some ways:

• Being able to test it without saving the map beforehand
• Returning to the contenteditor with the map opened again so you can continue editing.
• Add the inventory window to the testing environment so you can check containers.