Update freeze tests:

Changes:
* Don't skip test if TxFlags.h omits freeze flags
* Always run enforcement tests
* Mark passing book_offers filtering tests as broken
* Add skipped tests asserting old behaviour

I opened a pull request a while back to turn on the freeze enforcement tests but it was closed (paused), and I forgot about it. The tests all pass except for some related to book_offers filtering rules, which were specified at the time of writing. 

The broken tests in short:

1.  book_offers(pay=*, get=$frozen_account) // should show offers owned by $frozen_account only
2.  book_offers(pay=$frozen_account, get=*) // should show no offers

Reviewers: 
@JoelKatz  (freeze logic)
@vinniefalco (no more TxFlags.h check)
@miguelportilla (affects book_offers)