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

{
    BOOST_CHECK(TimingResistantEqual(std::string(""), std::string("")));
    BOOST_CHECK(!TimingResistantEqual(std::string("abc"), std::string("")));
    BOOST_CHECK(!TimingResistantEqual(std::string(""), std::string("abc")));
    BOOST_CHECK(!TimingResistantEqual(std::string("a"), std::string("aa")));
    BOOST_CHECK(!TimingResistantEqual(std::string("aa"), std::string("a")));
    BOOST_CHECK(TimingResistantEqual(std::string("abc"), std::string("abc")));
    BOOST_CHECK(!TimingResistantEqual(std::string("abc"), std::string("aba")));
}
/* Test strprintf formatting directives.
 * Put a string before and after to ensure sanity of element sizes on stack. */
#define B "check_prefix"
#define E "check_postfix"
BOOST_AUTO_TEST_CASE(strprintf_numbers)