txscript: Update signing tests to use params var.
txscript: Update signing tests to use params var.

This updates the script signing test code to use a single chain params
variable instead of repeatedly referring to a specific network.  This
makes it much easier to change in the future such as future testnet
resets.

jrick/dcrd,jcmincke/dcrd,jrick/btcd,decred/dcrd,jrick/btcd,jrick/btcd,jcmincke/dcrd,jcmincke/dcrd,jrick/dcrd,decred/dcrd