more clear randomness API for BABE (#8180)

* more clear randomness API for BABE

* babe: move randomness utilities to its own file

* node: use babe::RandomnessFromOneEpochAgo in random_seed implementation

* frame-support: annotate randomness trait with block number

* pallet-randomness-collective-flip: fix for new randomness trait

* pallet-society: fix randomness usage

* pallet-lottery: fix randomness usage

* pallet-contracts: fix randomness usage

* pallet-babe: fix randomness usage

we need to track when the current and previous epoch started so that we
know the block number by each existing on-chain was known

* node: fix random_seed

* node-template: fix random_seed

* frame-support: extend docs

* babe: add test for epoch starting block number tracking

* babe: fix epoch randomness docs

* frame: add todos for dealing with randomness api changes

Co-authored-by: André Silva <andrerfosilva@gmail.com>

substrate/frame/support/test/src/lib.rs

@@ -53,3 +53,21 @@ impl frame_support::traits::PalletInfo for PanicPalletInfo {
 		unimplemented!("PanicPalletInfo mustn't be triggered by tests");
 	}
 }
+
+/// Provides an implementation of [`Randomness`] that should only be used in tests!
+pub struct TestRandomness<T>(sp_std::marker::PhantomData<T>);
+
+impl<Output: codec::Decode + Default, T> frame_support::traits::Randomness<Output, T::BlockNumber>
+	for TestRandomness<T>
+where
+	T: frame_system::Config,
+{
+	fn random(subject: &[u8]) -> (Output, T::BlockNumber) {
+		use sp_runtime::traits::TrailingZeroInput;
+
+		(
+			Output::decode(&mut TrailingZeroInput::new(subject)).unwrap_or_default(),
+			frame_system::Module::<T>::block_number(),
+		)
+	}
+}