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/unsigned.rs

@@ -624,7 +624,7 @@ impl<T: Config> Pallet<T> {
 		ensure!(
 			Self::queued_solution().map_or(true, |q: ReadySolution<_>| raw_solution
 				.score
-				.strict_threshold_better(q.score, T::SolutionImprovementThreshold::get())),
+				.strict_threshold_better(q.score, T::BetterUnsignedThreshold::get())),
 			Error::<T>::PreDispatchWeakSubmission,
 		);
 
@@ -1066,7 +1066,7 @@ mod tests {
 			.desired_targets(1)
 			.add_voter(7, 2, bounded_vec![10])
 			.add_voter(8, 5, bounded_vec![10])
-			.solution_improvement_threshold(Perbill::from_percent(50))
+			.better_unsigned_threshold(Perbill::from_percent(50))
 			.build_and_execute(|| {
 				roll_to(25);
 				assert!(MultiPhase::current_phase().is_unsigned());