Fuzz honest multi-instance with incremental network sizes

`FuzzHonestMultiInstance_AsyncAgreement` intermittently fails on CI,
most likely due to taking too long to complete a test.

To avoid intermittent failures:
* Decrease the instance count to 3K
* change the honest multi instance tests to cover incremental network
  sizes, up to 4
* add additional static corpus that failed the fuzz test

Fixes #251

test/multi_instance_test.go

@@ -86,10 +86,12 @@ func FuzzHonestMultiInstance_SyncAgreement(f *testing.F) {
 
 func FuzzHonestMultiInstance_AsyncAgreement(f *testing.F) {
 	const (
-		instanceCount = 4000
-		honestCount   = 5
+		instanceCount = 5000
+		honestCount   = 4
 	)
 	f.Add(-7)
+	f.Add(31)
+	f.Add(33)
 	f.Fuzz(func(t *testing.T, seed int) {
 		multiAgreementTest(t, seed, honestCount, instanceCount, maxRounds*2, asyncOptions(seed)...)
 	})