fuzz: Script validation flags

Summary:
Add a fuzzer test for the validation flags.

Completes backport of core PR15399 (commit fab15ff):
https://github.com/bitcoin/bitcoin/pull/15399/commits/fab15ff70e8db26942db83a9aab7ebf974e8b6b2

Depends on D4628.

Test Plan:

./test/fuzz/test_runner.py -l DEBUG <path_to_your_corpus> script_flags

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4629