***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.
UniValue valRequest;
        if (!valRequest.read(req->ReadBody()))
            throw JSONRPCError(RPC_PARSE_ERROR, "Parse error");
        
        jreq.URI = req->GetURI();
        std::string strReply;
        
        if (valRequest.isObject()) {
            jreq.parse(valRequest);
            UniValue result = tableRPC.execute(jreq);
            
            strReply = JSONRPCReply(result, NullUniValue, jreq.id);
        
        } else if (valRequest.isArray())
            strReply = JSONRPCExecBatch(jreq, valRequest.get_array());
        else
            throw JSONRPCError(RPC_PARSE_ERROR, "Top-level object parse error");
        req->WriteHeader("Content-Type", "application/json");
        req->WriteReply(HTTP_OK, strReply);
    } catch (const UniValue& objError) {
        JSONErrorReply(req, objError, jreq.id);
        return false;
    } catch (const std::exception& e) {
        JSONErrorReply(req, JSONRPCError(RPC_PARSE_ERROR, e.what()), jreq.id);
        return false;
    }
    return true;
}
static bool InitRPCAuthentication()