test: Check submitblock return values

Summary:

Add assert_equal in some tests to check the submitblock return value

This is a backport of Core PR18745

Test Plan: test/functional/test_runner.py feature_bip68_sequence

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8965