***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.
/**
 * Type-check arguments; throws JSONRPCError if wrong type given. Does not check that
 * the right number of arguments are passed, just that any passed are the correct type.
 */
void RPCTypeCheck(const UniValue& params,
                  const std::list<UniValue::VType>& typesExpected, bool fAllowNull=false);
/**
 * Type-check one argument; throws JSONRPCError if wrong type given.
 */
void RPCTypeCheckArgument(const UniValue& value, UniValue::VType typeExpected);
/*
  Check for expected keys/value types in an Object.
*/
void RPCTypeCheckObj(const UniValue& o,
    const std::map<std::string, UniValueType>& typesExpected,
    bool fAllowNull = false,
    bool fStrict = false);
/** Opaque base class for timers returned by NewTimerFunc.
 * This provides no methods at the moment, but makes sure that delete
 * cleans up the whole state.
 */
class RPCTimerBase
{
public:
    virtual ~RPCTimerBase() {}
};
/**
 * RPC timer "driver".
 */
class RPCTimerInterface