values smaller than 0.001 sat/B can't be represented by CFeeRate, but they are not rejected.

```
./src/bitcoin-cli -named sendtoaddress address=bcrt1qwkmhqum095zau5rf2velq55cq938vrd7uw8m4r amount=1.23 fee_rate=0.0009

error code: -6
error message:
Fee rate (0.000 sat/vB) is lower than the minimum fee rate setting (1.000 sat/vB)
```

```
./src/bitcoin-cli -named sendtoaddress address=bcrt1qwkmhqum095zau5rf2velq55cq938vrd7uw8m4r amount=1.23 fee_rate=0.000000000009

error code: -3
error message:
Invalid amount

{
    const char *str_config =
       "a=\n"
       "b=1\n"
       "ccc=argument\n"
       "ccc=multiple\n"
       "d=e\n"
       "nofff=1\n"
       "noggg=0\n"
       "h=1\n"
       "noh=1\n"
       "noi=1\n"
       "i=1\n"
       "sec1.ccc=extend1\n"
       "\n"
       "[sec1]\n"
       "ccc=extend2\n"
       "d=eee\n"
       "h=1\n"
       "[sec2]\n"
       "ccc=extend3\n"
       "iii=2\n";
    TestArgsManager test_args;
    LOCK(test_args.cs_args);
    const auto a = std::make_pair("-a", ArgsManager::ALLOW_ANY);
    const auto b = std::make_pair("-b", ArgsManager::ALLOW_ANY);
    const auto ccc = std::make_pair("-ccc", ArgsManager::ALLOW_ANY);