[test] Make test_getpeerinfo more robust by using mocktime

Summary:
See title.

Depends on D17529

Test Plan:
Run rpc_net multiple times to make sure it isn't flaky:

for X in {1..30} ; do ./test/functional/test_runner.py rpc_net || break ; done

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17538