miner: Avoid stack-use-after-return in validationinterface

Summary:
This is a squash of 2 commits, to add the unbroken test and the solution
at the same time, and a revert of D5140 (different solution to the same problem)

test: Add unregister_validation_interface_race test

https://github.com/bitcoin/bitcoin/pull/18742/commits/fab6d060ce5f580db538070beec1c5518c8c777c

miner: Avoid stack-use-after-return in validationinterface

This is achieved by switching to a shared_ptr.

Also, switch the validationinterfaces in the tests to use shared_ptrs
for the same reason.

https://github.com/bitcoin/bitcoin/pull/18742/commits/7777f2a4bb1f9d843bc50a4e35085cfbb2808780

This is a backport of Core PR18742 [3/3]

This replaces the solution in D5140
See comment https://reviews.bitcoinabc.org/D5140#124338

Depends on D9104

Test Plan:
Test suggested by this PR:

export ASAN_OPTIONS=detect_leaks=0
ninja && ninja check
while ./src/test/test_bitcoin -t validationinterface_tests/unregister_validation_interface_race --catch_system_errors=no  ; do true; done

Test from D5140:

CC=clang CXX=clang++ cmake -GNinja .. -DCMAKE_BUILD_TYPE=Debug
ninja check-extended

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9105