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

{
public:
    struct Expect {
        util::SettingsValue setting;
        bool default_string = false;
        bool default_int = false;
        bool default_bool = false;
        const char* string_value = nullptr;
        std::optional<int64_t> int_value;
        std::optional<bool> bool_value;
        std::optional<std::vector<std::string>> list_value;
        const char* error = nullptr;
        explicit Expect(util::SettingsValue s) : setting(std::move(s)) {}
        Expect& DefaultString() { default_string = true; return *this; }
        Expect& DefaultInt() { default_int = true; return *this; }
        Expect& DefaultBool() { default_bool = true; return *this; }