tests: Add fuzzing harness for CKey related functions

Summary:
Backport of core PR17972.

The segwit related parts have been removed, and a few cases for Schnorr
signatures added. The missing corpora change is not relevant for us.

Test Plan:

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

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D8250