Fix change_signers_info benchmark

In #978 a couple of extra checks were added to
ensure that parameters related to signing could only change so quickly. This ended up breaking one
of the benchmarks since the benchmark setup used a difference that was larger than the allowed
amount.

I've fixed that by reading the current values from storage (set at Genesis) and updating those by
the allowed amount.

pallets/parameters/src/benchmarking.rs

@@ -55,7 +55,18 @@ benchmarks! {
   change_signers_info {
     let origin = T::UpdateOrigin::try_successful_origin().unwrap();
     pallet_session::CurrentIndex::<T>::put(1);
-    let signer_info = SignersSize { total_signers: 5, threshold: 3, last_session_change: 1 };
+
+    let SignersSize {
+        threshold: old_threshold,
+        total_signers: old_total_signers,
+        last_session_change: old_last_session_change,
+    } = SignersInfo::<T>::get();
+
+    let signer_info = SignersSize {
+        total_signers: old_total_signers + 1,
+        threshold: old_threshold + 1,
+        last_session_change: old_last_session_change + 1,
+    };
   }: {
     assert_ok!(
       <Parameters<T>>::change_signers_info(origin, signer_info.total_signers, signer_info.threshold)