tests: Add fuzzing harnesses for classes CBlockHeader, CFeeRate and various functions

Summary:

Add fuzzing harnesses for classes CBlockHeader, CFeeRate and various
functions.

Backport of core PR18353.

The amount random generator and the feerate fuzzer have been slightly
adapted to match our codebase.

Test Plan:

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

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8967