frame epm: expose feasibility_check in MinerConfig (#13555)

* frame epm: expose feasibity_check in miner

The goal with this commit is to expose the `feasibity_check` such
that anyone that implements the `MinerConfig trait` can utilize it

* cleanup

* fix tests

substrate/bin/node/runtime/src/lib.rs

@@ -702,6 +702,7 @@ impl pallet_election_provider_multi_phase::MinerConfig for Runtime {
 	type Solution = NposSolution16;
 	type MaxVotesPerVoter =
 	<<Self as pallet_election_provider_multi_phase::Config>::DataProvider as ElectionDataProvider>::MaxVotesPerVoter;
+	type MaxWinners = MaxActiveValidators;
 
 	// The unsigned submissions have to respect the weight of the submit_unsigned call, thus their
 	// weight estimate function is wired to this call's weight.