[tests] Fix tx padding
[tests] Fix tx padding

Summary: The padding goes after OP_RETURN

Test Plan: Test pad_raw_tx with and without -acceptnonstdtxn

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2374

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