tests: Add fuzzing harness for CConnman

Summary:
Backport of core#20188

This backport is out of order so much has been refactored. Compare this patch against git blame of Core master to get an idea of what changed. There are too many changes to list here.

Test Plan:
Setup fuzzer and then:

ninja fuzz-connman
export FUZZ_TARGET=connman
${AFLPATH}/afl-fuzz -i ${DIR_FUZZ_IN}/${FUZZ_TARGET} -o ${AFLOUT}/${FUZZ_TARGET} -m280 -- src/test/fuzz/${FUZZ_TARGET}

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12711