[compiletime] GroupAddUnit must return `false` if the passed unit is already in the group

[GetLocalPlayer]

let g = CreateGroup()
let u = createUnit(Player(0), 'hfoo', vec2(0, 0), 0 .fromDeg())
GroupAddUnit(g, u).assertTrue()
GroupAddUnit(g, u).assertFalse() // <-- line 101

    FAILED assertion:
    Test failed: Expected <false>, Actual <true>
... when calling boolean_assertFalse(true) in GroupTests.wurst:101
... when calling testVararg() in GroupTests.wurst:86

[Frotty]

Thanks, could you make a PR for the stdlib to add the unit test?

[GetLocalPlayer]

Cannot due another issue.