***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.
{
    if (IsRPCRunning())
        return false;
    
    std::map<std::string, const CRPCCommand*>::const_iterator it = mapCommands.find(name);
    if (it != mapCommands.end())
        return false;
    mapCommands[name] = pcmd;
    return true;
}
bool StartRPC()
{
    LogPrint(BCLog::RPC, "Starting RPC\n");
    fRPCRunning = true;
    g_rpcSignals.Started();
    return true;
}
void InterruptRPC()
{
    LogPrint(BCLog::RPC, "Interrupting RPC\n");
    
    fRPCRunning = false;
}
void StopRPC()