b651ef7e1c submitheader: more directly test missing prev block header (Gregory Sanders)
1e7f741745 remove some magic mining constants in functional tests (Gregory Sanders)
Pull request description:
The fewer magic numbers the better.
Also more directly tested a `submitheader` case of bad previous blockhash.
Tree-SHA512: 52b01a6aa199fa909eea4e9e84409a901933e545724e33149cc4132c82168199fd678809b6d94d95c9ff6ad02238a9552363620d13b8beaa5d4b67ade9ef425c