***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.
boost::algorithm::split(vargNames, argNamePattern, boost::algorithm::is_any_of("|"));
        auto fr = argsIn.end();
        for (const std::string & argName : vargNames) {
            fr = argsIn.find(argName);
            if (fr != argsIn.end()) {
                break;
            }
        }
        if (fr != argsIn.end()) {
            for (int i = 0; i < hole; ++i) {
                
                
                
                out.params.push_back(UniValue());
            }
            hole = 0;
            out.params.push_back(*fr->second);
            argsIn.erase(fr);
        } else {
            hole += 1;
        }
    }
    
    if (!argsIn.empty()) {
        throw JSONRPCError(RPC_INVALID_PARAMETER, "Unknown named parameter " + argsIn.begin()->first);
    }
    
    return out;
}
UniValue CRPCTable::execute(const JSONRPCRequest &request) const