***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.
* @returns List of registered commands.
    */
    std::vector<std::string> listCommands() const;
    /**
     * Appends a CRPCCommand to the dispatch table.
     * Returns false if RPC server is already running (dump concurrency protection).
     * Commands cannot be overwritten (returns false).
     */
    bool appendCommand(const std::string& name, const CRPCCommand* pcmd);
};
bool IsDeprecatedRPCEnabled(const std::string& method);
extern CRPCTable tableRPC;
/**
 * Utilities: convert hex-encoded Values
 * (throws error if not hex).
 */
extern uint256 ParseHashV(const UniValue& v, std::string strName);
extern uint256 ParseHashO(const UniValue& o, std::string strKey);
extern std::vector<unsigned char> ParseHexV(const UniValue& v, std::string strName);
extern std::vector<unsigned char> ParseHexO(const UniValue& o, std::string strKey);
extern CAmount AmountFromValue(const UniValue& value);
extern std::string HelpExampleCli(const std::string& methodname, const std::string& args);
extern std::string HelpExampleRpc(const std::string& methodname, const std::string& args);
bool StartRPC();
void InterruptRPC();
void StopRPC();
std::string JSONRPCExecBatch(const JSONRPCRequest& jreq, const UniValue& vReq);
int RPCSerializationFlags();
#endif