Not really, testing the changes is rather simplistic.
There'd be a debug mode in the game for development. This can range from something as complex as floating damage markers like RPGs use or a few lines in the GUI all the way to a simple log that gets output and read after the fact. The code for weapon damage works, balancing them is just quite literally changing numbers. Sure, it may be a lot of trial and error but it's not complicated and not anywhere near as time consuming as adding in an entirely new network feature.
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It's easy but takes a long time. Checking if a payment was successfully is just checking it works, checking if everything is balanced is a longer process. Seen plenty of games where they boost a stay (in this example magic attack) didn't fully test it and mate became the meta.
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[quote]Checking if a payment was successfully is just checking it works[/quote]Obviously you ignored the entirety of the third paragraph.