fix incomplete txvalidationcache_tests

Summary:
This test was missed in the LCG updates (see D2475, D2481, D2482).
The lack of flag range was partly responsible for the problem behind D4451
being hidden until activation.

I've also removed the | MANDATORY from the flag scanning and restored the
original "Filter out incompatible flag choices" approach (reversing the
liberty taken in backport D534). Since we have so many mandatory flags
(soon to be even more) this makes sure the tests cover as many flag
combinations as possible. But, since we don't just have soft-forking flags,
it is important that tests can define the required_flags that they rely
on for function.

Test Plan: make check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4671