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(test_args.IsArgSet("-ccc"));
    BOOST_CHECK(test_args.IsArgSet("-d"));
    BOOST_CHECK(test_args.IsArgSet("-fff"));
    BOOST_CHECK(test_args.IsArgSet("-ggg"));
    BOOST_CHECK(test_args.IsArgSet("-h"));
    BOOST_CHECK(test_args.IsArgSet("-i"));
    BOOST_CHECK(!test_args.IsArgSet("-zzz"));
    BOOST_CHECK(!test_args.IsArgSet("-iii"));
    BOOST_CHECK_EQUAL(test_args.GetArg("-a", "xxx"), "");
    BOOST_CHECK_EQUAL(test_args.GetArg("-b", "xxx"), "1");
    BOOST_CHECK_EQUAL(test_args.GetArg("-ccc", "xxx"), "argument");
    BOOST_CHECK_EQUAL(test_args.GetArg("-d", "xxx"), "e");
    BOOST_CHECK_EQUAL(test_args.GetArg("-fff", "xxx"), "0");
    BOOST_CHECK_EQUAL(test_args.GetArg("-ggg", "xxx"), "1");
    BOOST_CHECK_EQUAL(test_args.GetArg("-h", "xxx"), "0");
    BOOST_CHECK_EQUAL(test_args.GetArg("-i", "xxx"), "1");
    BOOST_CHECK_EQUAL(test_args.GetArg("-zzz", "xxx"), "xxx");
    BOOST_CHECK_EQUAL(test_args.GetArg("-iii", "xxx"), "xxx");
    for (const bool def : {false, true}) {
        BOOST_CHECK(test_args.GetBoolArg("-a", def));
        BOOST_CHECK(test_args.GetBoolArg("-b", def));
        BOOST_CHECK(!test_args.GetBoolArg("-ccc", def));
        BOOST_CHECK(!test_args.GetBoolArg("-d", def));
        BOOST_CHECK(!test_args.GetBoolArg("-fff", def));
        BOOST_CHECK(test_args.GetBoolArg("-ggg", def));
        BOOST_CHECK(!test_args.GetBoolArg("-h", def));
        BOOST_CHECK(test_args.GetBoolArg("-i", def));
        BOOST_CHECK(test_args.GetBoolArg("-zzz", def) == def);
        BOOST_CHECK(test_args.GetBoolArg("-iii", def) == def);
    }