tests: Add fuzzing harness for CNode
Summary:
Backport of core PR19067.
Depends on D9192.
Test Plan:
ninja bitcoin-fuzzers ./src/test/fuzz/net <path_to_corpus>
Reviewers: #bitcoin_abc, majcosta
Reviewed By: #bitcoin_abc, majcosta
Differential Revision: https://reviews.bitcoinabc.org/D9193