x/gov/keeper: fix flaky TestPaginatedVotesQuery (#9223)

When testing with -race, sometimes the random source generate the same
string for consecutive calls, causing duplicated voter address. So the
number of votes in DB is not 20.

To fix this, we ensure unique addresses are generated, by using a map
for tracking which one was produced, and skip the duplicated address and
generated new one. Testing with:

	go test -race -v -count=1000 -run=TestPaginatedVotesQuery

now passes.

Updates #9010

x/gov/keeper/querier_test.go

@@ -316,13 +316,23 @@ func TestPaginatedVotesQuery(t *testing.T) {
 	app.GovKeeper.SetProposal(ctx, proposal)
 
 	votes := make([]types.Vote, 20)
-	rand := rand.New(rand.NewSource(time.Now().UnixNano()))
-	addr := make(sdk.AccAddress, 20)
+	random := rand.New(rand.NewSource(time.Now().UnixNano()))
+	addrMap := make(map[string]struct{})
+	genAddr := func() string {
+		addr := make(sdk.AccAddress, 20)
+		for {
+			random.Read(addr)
+			addrStr := addr.String()
+			if _, ok := addrMap[addrStr]; !ok {
+				addrMap[addrStr] = struct{}{}
+				return addrStr
+			}
+		}
+	}
 	for i := range votes {
-		rand.Read(addr)
 		vote := types.Vote{
 			ProposalId: proposal.ProposalId,
-			Voter:      addr.String(),
+			Voter:      genAddr(),
 			Options:    types.NewNonSplitVoteOption(types.OptionYes),
 		}
 		votes[i] = vote