317f2cb3f4 test: Check RPC settxfee errors (João Barbosa)
48618daf26 Add checks for settxfee reasonableness (Anthony Towns)
Pull request description:
When using the `settxfee` RPC, the value is silently ignored if it is less than either than minrelaytxfee or the wallet's mintxfee. This adds an error response if that's going to happen, but still allows "settxfee 0" to deliberately default to the minimum value.
Tree-SHA512: ce685584cf8d6b9ca2cc97196d494220e3892b6a804a458086e04b3a23df281da432ad0a3053106a064c90c541ddb6f6b96a27cf8376d45af1e44449baf88456