Currently, nExtraNonce is always initialized to zero
when mining a new block. However, this causes deterministic
nonces during functional and unit tests. In some cases, tests
which make use of the ABC block parking functionality will
mine two blocks on the same tip, with the same nonces. This
results in two blocks with the same hash, causing it to be
ignored when attempting to accept the block. The result of
this is that intentional reorgs in functional tests may not
occur when intended.
This commit changes the initialization of nExtraNonce to
be a random number so that no two calls to generatetoaddress
create identical blocks.