`reportUnusedFunction` should not consider tests

[KotlinIsland]

def _f(): ...

def test_f():
    _f()

i would expect this to report that _f() is unused, maybe this should be configurable? would need a way to specify what a test is

[DetachHead]

it should be configurable so it depends on #539, and when running in "application" mode (ie. not a library), it should also report unused public functions as well as not count usages in tests. this functionality would also need to rely on #311 i guess