fuzz: Initialize a full TestingSetup where appropriate

Summary:
A full TestingSetup is required for both coins_view and
load_external_block_file as they interact with the active chainstate.

This is a backport of core#20946 [2/2]
https://github.com/bitcoin/bitcoin/pull/20946/commits/abb6fa728598c4cc8874eae1c3c5e587e36424cd

Depends on D11678

Test Plan: ninja bitcoin-fuzzers

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11679