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

int processstatus;
    BOOST_CHECK_EQUAL(write(fd[1], &LockCommand, 1), 1);
    BOOST_CHECK_EQUAL(write(fd[1], &ExitCommand, 1), 1);
    BOOST_CHECK_EQUAL(waitpid(pid, &processstatus, 0), pid);
    BOOST_CHECK_EQUAL(processstatus, 0);
    BOOST_CHECK_EQUAL(LockDirectory(dirname, lockname, true), true);
    
    signal(SIGCHLD, old_handler);
    BOOST_CHECK_EQUAL(close(fd[1]), 0); 
#endif
    
    ReleaseDirectoryLocks();
    fs::remove_all(dirname);
}
BOOST_AUTO_TEST_CASE(test_DirIsWritable)
{
    
    fs::path tmpdirname = m_args.GetDataDirPath();
    BOOST_CHECK_EQUAL(DirIsWritable(tmpdirname), true);
    
    tmpdirname = GetUniquePath(tmpdirname);
    BOOST_CHECK_EQUAL(DirIsWritable(tmpdirname), false);
    fs::create_directory(tmpdirname);
    
    BOOST_CHECK_EQUAL(DirIsWritable(tmpdirname), true);
    fs::remove(tmpdirname);
}
BOOST_AUTO_TEST_CASE(test_ToLower)