***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.
{
public:
    std::string category;
    std::string name;
    rpcfn_type actor;
    std::vector<std::string> argNames;
};
/**
 * Bitcoin RPC command dispatcher.
 */
class CRPCTable
{
private:
    std::map<std::string, const CRPCCommand*> mapCommands;
public:
    CRPCTable();
    const CRPCCommand* operator[](const std::string& name) const;
    std::string help(const std::string& name, const JSONRPCRequest& helpreq) const;
    /**
     * Execute a method.
     * @param request The JSONRPCRequest to execute
     * @returns Result of the call.
     * @throws an exception (UniValue) when an error happens.
     */
    UniValue execute(const JSONRPCRequest &request) const;
    /**
    * Returns a list of registered commands