The genesis blocks are not really part of the wire protocol rather they
are part of a network parameters. Thus, this commit moves the all of the
gensis blocks and tests to the btcnet package.
Also, create variables in the test package for the mainnet genesis hash,
merkle root, and coinbase transaction for use throughout the tests since
they the exported values are no longer available.
The go vet command complains about untagged struct initializers when
defining a ShaHash directly. This seems to be a limitation where go vet
does not exclude the warning for types which are a constant size byte array
like it does for normal constant size byte array definition.
This commit simply modifies the tests to use a constant definition cast to a
ShaHash to overcome the limitation of go vet.
