Async environment for mods to do concurrent tasks (#11131)

games/devtest/mods/unittests/inside_async_env.lua

@@ -0,0 +1,15 @@
+unittests = {}
+
+core.log("info", "Hello World")
+
+function unittests.async_test()
+	assert(core == minetest)
+	-- stuff that should not be here
+	assert(not core.get_player_by_name)
+	assert(not core.set_node)
+	assert(not core.object_refs)
+	-- stuff that should be here
+	assert(ItemStack)
+	assert(core.registered_items[""])
+	return true
+end