[pallet_author_inherent] `kick_off_authorship_validation` benchmark fails

[Chralt98]

It fails here. The T::SlotBeacon::slot() seems to return zero, although it was set in the benchmarks here.

set_block_number has an empty default implementation here.
So I assume the problem here is, that set_block_number was not properly implemented on the moonbeam nimbus side.

This is useful in case the block number provider is different than System

So the fix is to implement set_block_number for BlockNumberProvider:

impl BlockNumberProvider {
      type BlockNumber = u32;

      fn current_block_number() -> Self {
	      frame_system::Pallet<Config>::block_number()
      }

      #[cfg(features = "runtime-benchmarks")]
      fn set_block_number(slot: u32) -> Self {
	      frame_system::Pallet<Config>::set_block_number(slot);
      }
}