In this commit, we modify GetTestTxidAndScript to generate new P2PKH
scripts. This is needed to properly test confirmations and spends of
unique scripts on-chain within the set of interface-level test cases.
In this commit, we extract some of the helper test variables and
functions into their own file and guard them under a build flag. This is
needed as some unit tests will be introduced in a future commit where
most of the same functions within the interface tests are reused. In
order to prevent these variables and functions from being exportable, we
guard them by the "debug" build tag.
