***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.
{
    unsigned int vcidx;
    for (vcidx = 0; vcidx < (sizeof(vRPCCommands) / sizeof(vRPCCommands[0])); vcidx++)
    {
        const CRPCCommand *pcmd;
        pcmd = &vRPCCommands[vcidx];
        mapCommands[pcmd->name] = pcmd;
    }
}
const CRPCCommand *CRPCTable::operator[](const std::string &name) const
{
    std::map<std::string, const CRPCCommand*>::const_iterator it = mapCommands.find(name);
    if (it == mapCommands.end())
        return nullptr;
    return (*it).second;
}
bool CRPCTable::appendCommand(const std::string& name, const CRPCCommand* pcmd)