Make `BlockNumberProvider::set_block_number` available in tests / std (#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.

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Co-authored-by: Bastian Köcher <[email protected]>
Co-authored-by: Oliver Tale-Yazdi <[email protected]>