tests: Add fuzzing harness for AS-mapping (asmap)

Summary:
Backport of core PR18029.

The missing corpora change is not relevant for us.
The netaddress.h change was already done in D8200.

Test Plan:

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

Reviewers: #bitcoin_abc, PiRK

Reviewed By: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8216