test: Check submitblock return values
test: Check submitblock return values

Summary:
> Add assert_equal in some tests to check the submitblock return value

This is a backport of Core [[https://github.com/bitcoin/bitcoin/pull/18745 | 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

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