test: increase key_gen_ecdsa_p256 rounds

This number of iterations includes the first generated key that happens to have a leading zero byte in its secret MPI.

src/composed/key/builder.rs

@@ -624,7 +624,7 @@ mod tests {
     #[test]
     fn key_gen_ecdsa_p256() {
         let rng = &mut ChaCha8Rng::seed_from_u64(0);
-        for _ in 0..100 {
+        for _ in 0..=175 {
             gen_ecdsa(rng, ECCCurve::P256);
         }
     }