fuzz: Initialize a TestingSetup for test_one_input

Summary:
For fuzz tests that need it.

This is a backport of core#21866 [6/12]
https://github.com/bitcoin/bitcoin/pull/21866/commits/ee0ab1e959e0e75e04d87fabae8334ad4656f3e5

Note: src/test/fuzz/validation_load_mempool.cpp has not been created yet in Bitcoin ABC. If/When it will eventually be backported, it will be obvious that this change is needed, as g_chainman will no longer exist by then.

Test Plan: ninja bitcoin-fuzzers

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11682