As suggested by Greg Maxwell-- unit test to make sure a block
with a double-spend in it doesn't pass validation if half of
the double-spend is already in the memory pool (so full-blown
transaction validation is skipped) when the block is received.
Make sure that chainparams and logging is properly initialized. Doing
this for every test may be overkill, but this initialization is so
simple that that does not matter.
This should fix the travis issues.
