***UPDATE:***

Turns out the issue was caused not by testnet, but by having 2 wallets @ryanofsky found out !

----------

Step to reproduce:

This work on regtest just fine:
```
bitcoind -wallet=test -regtest -rpcpassword=abc -rpcuser=abc
```

Try to make a batch RPC request to /wallet/test:

```
[{"jsonrpc":"1.0","id":1,"method":"getbalance","params":["*"]}]
```
You get the expected result.

However; same things with testnet:
```
bitcoind -wallet=test -testnet -rpcpassword=abc -rpcuser=abc
```

Result:

"Wallet file not specified (must request wallet RPC through /wallet/<filename> uri-path)."

This problem is limited to batched RPC request on testnet as just making the exact same request without the '[' and ']' works fine. It works also fine on regtest, which is crazy. I will look into later.
#ifndef BITCOIN_RPCSERVER_H
#define BITCOIN_RPCSERVER_H
#include "amount.h"
#include "rpc/protocol.h"
#include "uint256.h"
#include <list>
#include <map>
#include <stdint.h>
#include <string>
#include <univalue.h>
static const unsigned int DEFAULT_RPC_SERIALIZE_VERSION = 1;
class CRPCCommand;
namespace RPCServer
{
    void OnStarted(std::function<void ()> slot);
    void OnStopped(std::function<void ()> slot);
}
/** Wrapper for UniValue::VType, which includes typeAny:
 * Used to denote don't care type. Only used by RPCTypeCheckObj */
struct UniValueType {
    explicit UniValueType(UniValue::VType _type) : typeAny(false), type(_type) {}
    UniValueType() : typeAny(true) {}
    bool typeAny;
    UniValue::VType type;
};
class JSONRPCRequest