tests: Add fuzzing harnesses for CAutoFile, CBufferedFile, LoadExternalBlockFile and other FILE* consumers

Summary:
Backport of core PR19143.

Depends on D9131.

The last commit
(https://github.com/bitcoin/bitcoin/pull/19143/commits/ad6c34881dc125c973b6b9ba1daa999d3141b1ae)
has been skipped since we don't have the policy estimator like core.

Test Plan:

ninja bitcoin-fuzzers
./test/fuzz/test_runner.py <path_to_corpus>

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9132