dcrutil: Use mock addr params in tests.
dcrutil: Use mock addr params in tests.

This implements a mock address params struct that implements the new
interface and modifies the test code to make use of the mock params
instead of chaincfg.  This completely decouples the tests, with the
exception of the example code, from chaincfg and therefore any updates
to it such as introducing new test networks will not require changes to
the tests.

jrick/dcrd,decred/dcrd,decred/dcrd,jrick/dcrd