Check argument against negative sleep duration

crates/lb/tests/task.lua

@@ -111,6 +111,11 @@ describe("spawn", function()
 end)
 
 describe("sleep", function()
+  test("invalid arg", function()
+    assert(not pcall(task.sleep, "invalid"))
+    task.sleep(-1) -- negative sleep should just become 0ms
+  end)
+
   test("sleep is asynchronous", function()
     local value
     spawn(function()