Convert a bunch of the helper functions to instead be methods on the
testCtx type. This is in preparation for adding a mockChain to the
testCtx that these helpers can then use to add blocks and utxos to.
See `notifications_test.go` for an idea of what we are trying to emulate
here. Once the funding tx code has moved to the gossiper, then the logic
in `notifications_test.go` will be removed.