Function invoke_pt_cache 

pub fn invoke_pt_cache(nub: &Nub, top: &PtCacheTop, n: u64) -> (u64, u64)

One page-table-cache bench iteration: invoke endpoint 0, which host_calls the resident B n times. Asserts a clean trampoline halt and that the summed echoes match. Returns (return_value, gas_used).