***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.
#include "httprpc.h"
#include "base58.h"
#include "chainparams.h"
#include "httpserver.h"
#include "rpc/protocol.h"
#include "rpc/server.h"
#include "random.h"
#include "sync.h"
#include "util.h"
#include "utilstrencodings.h"
#include "ui_interface.h"
#include "crypto/hmac_sha256.h"
#include <stdio.h>
#include <boost/algorithm/string.hpp> 
static const char* WWW_AUTH_HEADER_DATA = "Basic realm=\"jsonrpc\"";
/** Simple one-shot callback timer to be used by the RPC mechanism to e.g.
 * re-lock the wallet.
 */
class HTTPRPCTimer : public RPCTimerBase