In-game testing, test environment and the debug menu
Whether you are implementing a new feature or whether you are fixing a bug, it is always a good practice to test your changes in-game. It can be a hard task to create the exact conditions by playing a normal game to be able to test your changes, which is why there is a debug menu. There is no default key to bring up the menu so you will need to assign one first.
Bring up the keybindings menu (press Escape then 1), scroll down almost to the bottom and press + to add a new key binding. Press the letter that corresponds to the Debug menu item, then press the key you want to use to bring up the debug menu. To test your changes, create a new world with a new character. Once you are in that world, press the key you just assigned for the debug menu and you should see something like this:
┌────────────────────────────────────────────┐
│ Debug Functions - Using these is CHEATING! │
├────────────────────────────────────────────┤
│ 1 Wish for an item │
│ 2 Teleport - Short Range │
│ 3 Teleport - Long Range │
│ 4 Reveal map │
│ 5 Spawn NPC │
│ 6 Spawn Monster │
│ 7 Check game state... │
│ 8 Kill NPCs │
│ 9 Mutate │
│ 0 Spawn a vehicle │
│ a Change all skills │
│ b Learn all melee styles │
│ c Unlock all recipes │
│ d Edit player/NPC │
│ e Spawn Artifact │
│ f Spawn Clairvoyance Artifact │
│ g Map editor │
│ h Change weather │
│ i Remove all monsters │
│ j Display hordes │
│ k Test Item Group │
│ l Damage Self │
│ m Show Sound Clustering │
│ n Lua Command │
│ o Display weather │
│ p Change time │
│ q Set automove route │
│ r Show mutation category levels │
│ s Cancel │
└────────────────────────────────────────────┘
With these commands, you should be able to recreate the proper conditions to test your changes. You can find some more information about the debug menu on the official wiki.