[Mauzen]

Tests always are a good idea. Though I dont see that many applications for them in gamemodes, as most functions dont return stuff, but do something ingame. Testing ingame stuff often is overkill, and most of the returning functions are too simple to justify writing a test for them.
For me, testing in samp would probably be limited to just a handful of funcitons deep down at the low level of "mechanical" functions.