***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 (strUserPass.find(":") == std::string::npos) {
        return false;
    }
    std::string strUser = strUserPass.substr(0, strUserPass.find(":"));
    std::string strPass = strUserPass.substr(strUserPass.find(":") + 1);
    for (const std::string& strRPCAuth : gArgs.GetArgs("-rpcauth")) {
        
        std::vector<std::string> vFields;
        boost::split(vFields, strRPCAuth, boost::is_any_of(":$"));
        if (vFields.size() != 3) {
            
            continue;
        }
        std::string strName = vFields[0];
        if (!TimingResistantEqual(strName, strUser)) {
            continue;
        }
        std::string strSalt = vFields[1];