test: Set peertimeout in write_config

Summary:
This avoids having to remember to set it whenever mocktime is used with
peer connections. Also, it might help avoiding disconnects when
attaching a debugger to a running test.

This is a backport of core#22960
Depends on D10955

Test Plan:
ninja check-functional-extended
grep -r peertimeout test/functional/

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10956