Tests: Use self.chain instead of 'regtest' in all current tests

Summary:
This is a backport of Core PR16681 and PR18069

I also replaced one occurence of 'regtest' in test_framework.py, that was missed in the backport of PR16509 (D5942)

Test Plan: ninja && ninja check-functional-extended

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8738