test: Move main_tests to validation_tests

Summary:
Partial backport of Core PR15504: https://github.com/bitcoin/bitcoin/pull/15504/commits/fa85468cd252f51ad5328272a1bc19b7517e204e

validation_tests.cpp was already added, but those changes do not overlap with Core: D189

Test Plan:

ninja check
make check

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4738