Split SolutionImprovementThresholds into two types (#11221)

* Splitting `SolutionImprovementThreshold` in 2
One for Signed phase and one for Unsigned phase.

* Adding some tests

* Fixes after code review.
- Removing `GetDefault`.
- Shorter naming.
- More explicit test.

substrate/frame/election-provider-multi-phase/src/lib.rs

@@ -102,8 +102,8 @@
 //! valid if propagated, and it acts similar to an inherent.
 //!
 //! Validators will only submit solutions if the one that they have computed is sufficiently better
-//! than the best queued one (see [`pallet::Config::SolutionImprovementThreshold`]) and will limit
-//! the weight of the solution to [`pallet::Config::MinerMaxWeight`].
+//! than the best queued one (see [`pallet::Config::BetterUnsignedThreshold`]) and will limit the
+//! weight of the solution to [`pallet::Config::MinerMaxWeight`].
 //!
 //! The unsigned phase can be made passive depending on how the previous signed phase went, by
 //! setting the first inner value of [`Phase`] to `false`. For now, the signed phase is always
@@ -585,9 +585,14 @@ pub mod pallet {
 		type SignedPhase: Get<Self::BlockNumber>;
 
 		/// The minimum amount of improvement to the solution score that defines a solution as
-		/// "better" (in any phase).
+		/// "better" in the Signed phase.
 		#[pallet::constant]
-		type SolutionImprovementThreshold: Get<Perbill>;
+		type BetterSignedThreshold: Get<Perbill>;
+
+		/// The minimum amount of improvement to the solution score that defines a solution as
+		/// "better" in the Unsigned phase.
+		#[pallet::constant]
+		type BetterUnsignedThreshold: Get<Perbill>;
 
 		/// The repeat threshold of the offchain worker.
 		///