fuzz: Improve fuzzing stability for txorphan harness

Summary:
Backport of core#29031.

Note that we don't have the fuzz test backported yet, the point is to bring the file up to date with upstream.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16506