[backport#17357 2/2] tests: Add fuzzing harness for Bech32 encoding/decoding

Summary:
This ended up a bit different from Core's for obvious reasons

https://github.com/bitcoin/bitcoin/pull/17357/commits/b7541705d0abfbddf682a0134f3fa8a8e1d06cdf

Depends on D6904

Concludes backport of Core PR17357

Test Plan:

cmake -GNinja .. -DENABLE_SANITIZERS="address;fuzzer" -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++
ninja bitcoin-fuzzers
./src/test/fuzz/cashaddr

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6906