refactor: TestTxPool_ConcurrentTxs (#1503)

Attempts to close
#1194

## Testing

It takes a while to run so I can't run it with a super high count. 

```
$ go test ./... -run "TestTxPool_ConcurrentTxs" -count=10
ok  	github.com/tendermint/tendermint/mempool/cat	30.391s
```