Builtin clock function not fully tested

[xtreme-james-cooper]

I accidentally introduced a regression in my jlox interpreter - the one builtin function, clock, could take arbitrary arguments. I didn't notice for a bit since the jlox suite kept passing. The fix is pretty simple: add a test (in tests/function? in tests/regression?) to ensure clock errors if it is given arguments.

clock_args.lox:
clock(0); // expect runtime error: Expected 0 arguments but got 1.