[unit test] package validation subsets and subpackages

- Interdependent parents.
- Allow normal RBF for individual transactions that don't require
  in-package CPFP.
- Diamond where bottom of package is low feerate.
- Package where there is only room for 1, so linearization helps us pick
  the highest feerate one.
- RBF within package. Even though package RBF is not enabled, it's
  important that submitting a transaction as part of a package does not
  block it from doing a normal replacement. Otherwise we may blind
  ourselves to a replacement simply because it has a child and we happened
  to download it as a package.

Co-authored-by: Andrew Chow <achow101@gmail.com>