***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:
    UniValue id;
    std::string strMethod;
    UniValue params;
    bool fHelp;
    std::string URI;
    std::string authUser;
    JSONRPCRequest() : id(NullUniValue), params(NullUniValue), fHelp(false) {}
    void parse(const UniValue& valRequest);
};
bool IsRPCRunning();
/**
 * Set the RPC warmup status.  When this is done, all RPC calls will error out
 * immediately with RPC_IN_WARMUP.
 */
void SetRPCWarmupStatus(const std::string& newStatus);
void SetRPCWarmupFinished();
bool RPCIsInWarmup(std::string *outStatus);