I was wonder how I would make a "cheat" or something for a specific player. Say, a unit with 9999 hp and attack, or a flying unit with the move speed of 100 for testing purposes. However, I only want a specific player to use it. Maybe it will be the map maker, maybe a friend. (another thought: If a map tester was particularly helpful, instead of him spawning a marine called "marine", the marine will be called "flashbeer" with the exact same stats; however, only for that player if he/she is in the game.) This should not be some type of stupid code that the player has to type in or task they have to perform.
How could I do this?

Something I thought of were identifying the player username and/or code and/or ip address. However, I have no idea how to do this or if it is even possible.

You can get the player name of a player, yes.
But I cannot see a function that makes a string out of text...

You can still use buttonpresses and/or passwords for this...

It is not currently possible to use the player's name for anything useful, only for display. You will need to rely on button presses or passwords as Ahli suggested.

Converting usernames into strings is something blizzard was looking at doing, so It might work after that is implmented, but for now that isn't possible, only through what Ahli and Temp have already said