***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.
#include "rpc/server.h"
#include "base58.h"
#include "fs.h"
#include "init.h"
#include "random.h"
#include "sync.h"
#include "ui_interface.h"
#include "util.h"
#include "utilstrencodings.h"
#include <univalue.h>
#include <boost/bind.hpp>
#include <boost/signals2/signal.hpp>
#include <boost/algorithm/string/case_conv.hpp> 
#include <boost/algorithm/string/classification.hpp>
#include <boost/algorithm/string/split.hpp>
#include <memory> 
#include <unordered_map>
static bool fRPCRunning = false;
static bool fRPCInWarmup = true;
static std::string rpcWarmupStatus("RPC server started");
static CCriticalSection cs_rpcWarmup;
static RPCTimerInterface* timerInterface = nullptr;
static std::map<std::string, std::unique_ptr<RPCTimerBase> > deadlineTimers;
static struct CRPCSignals
{
    boost::signals2::signal<void ()> Started;
    boost::signals2::signal<void ()> Stopped;
    boost::signals2::signal<void (const CRPCCommand&)> PreCommand;
} g_rpcSignals;
void RPCServer::OnStarted(std::function<void ()> slot)