Changed the time period covered for DoS tests from 4 days to 3, to take into account massively reduced retarget time from DigiShield.
Rewrote tests for mining rewards, to at least approximately match Doge.
Due to huge number of DOGE, tests cannot be completed without significant
re-engineering of number handling code.
Merge pull request #448 from rnicoll/1.7-dev-test-main

Fix mining rewards test
Merge pull request #447 from rnicoll/1.7-dev-tests

Fix DoS tests to take into account DigiShield.
Suck it Gitian!