btcjson: Specify constant size in tests.

This modifies the test for createrawtransaction to specify the constant
size passed as an int64.  This is necessary because the NewCmd function
accepts the parameters as interfaces in order to support varargs and a
raw numeric constant is treated as an integer.  Since the constant value
is larger than an int32, this causes certain platforms like ARM which
treat a raw integer as a 32-bit integer to fail to compile.

This is work towards #600.

btcjson/chainsvrcmds_test.go

@@ -63,7 +63,7 @@ func TestChainSvrCmds(t *testing.T) {
 			name: "createrawtransaction optional",
 			newCmd: func() (interface{}, error) {
 				return btcjson.NewCmd("createrawtransaction", `[{"txid":"123","vout":1}]`,
-					`{"456":0.0123}`, 12312333333)
+					`{"456":0.0123}`, int64(12312333333))
 			},
 			staticCmd: func() interface{} {
 				txInputs := []btcjson.TransactionInput{