Make BlockNumberProvider::set_block_number available in tests / std (…

…paritytech#3974)

This function is currently only exposed to runtime-benchmarks, where it
should be available in all tests.

Also made it available in `std` because this is how `set_block_number`
works in FRAME System, however, not sure if it is needed in the latest
std/no_std paradigm.

---------

Co-authored-by: Bastian Köcher <git@kchr.de>
Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

substrate/primitives/runtime/src/traits.rs

@@ -2322,12 +2322,12 @@ pub trait BlockNumberProvider {
  /// .
  fn current_block_number() -> Self::BlockNumber;
 
- /// Utility function only to be used in benchmarking scenarios, to be implemented optionally,
- /// else a noop.
+ /// Utility function only to be used in benchmarking scenarios or tests, to be implemented
+ /// optionally, else a noop.
  ///
  /// It allows for setting the block number that will later be fetched
  /// This is useful in case the block number provider is different than System
- #[cfg(feature = "runtime-benchmarks")]
+ #[cfg(any(feature = "std", feature = "runtime-benchmarks"))]
  fn set_block_number(_block: Self::BlockNumber) {}
 }