Fix TestProber4 test case
Fix TestProber4 test case

Earlier logic kept p.Alerting == true if a p.Probe() run for a
silenced probe, even though p.Badness was reset to 0. After
introducing the RWMutex to guard the state in p.alerting, we now
correctly set it to false in this situation. This commit updates the
test to match the fixed behavior.

hkjn/prober