validation tests: Use existing {Chainstate,Block}Man

Summary:
Use {Chain,}TestingSetup's existing {Chainstate,Block}Manager and avoid
unnecessarily creating a local one.

This also helps reduce the code diff for a later commit where we change
{Chainstate,Block}Manager's constructor signature.

This is a backport of core#25815

Test Plan: ninja check

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D16004