Fix #22 - manage systems before setSystemIndex

spec/tiny_spec.lua

@@ -310,4 +310,14 @@ describe('tiny-ecs:', function()
         assert.are.same(_G, GLOBALS)
     end)
 
+    it("Can set system indices", function()
+        local world = tiny.world()
+        local systemA = tiny.system()
+        local systemB = tiny.system()
+        world:addSystem(systemA)
+        world:addSystem(systemB)
+        world:setSystemIndex(systemA, 1)
+        assert(true)
+    end)
+
 end)