Test last_probe_time_should_advance is flakey

[ximon18]

From: https://github.com/NLnetLabs/krill/actions/runs/6655929088/job/18110471770#step:5:1005

---- commons::crypto::signing::signers::probe::tests::last_probe_time_should_advance stdout ----
Error: thread 'main' panicked at 'assertion failed: t2 == t1', src/commons/crypto/signing/signers/probe.rs:355:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

This failed in a test run in which the change made seems unrelated to the failure.

[ximon18]

The durations used in the test were increased recently to work around this, but a proper fix will require removing the dependence on elapsed time.