Fix issue not all signers in certificate test end to end

This happens from time to time, when the first signer wins all the lotteries,
the stm deduplication of signatures keeps only signatures from the first signer.
Reducing the f parameter limits the possibility of such an occurrence.

Author: jpraynaud

mithril-test-lab/mithril-end-to-end/src/end_to_end_spec.rs

@@ -246,7 +246,7 @@ async fn update_protocol_parameters(aggregator: &mut Aggregator) -> Result<(), S
     let protocol_parameters_new = ProtocolParameters {
         k: 150,
         m: 200,
-        phi_f: 0.95,
+        phi_f: 0.80,
     };
     info!(
         "> updating protocol parameters to {:?}...",