res_sorcery_memory_cache: Add test event when a refresh occurs.

This change adds a testsuite event for when a refresh occurs.
This is useful as it provides a guaranteed mechanism of knowing when
it has occurred instead of waiting an arbitrary amount of time.

ASTERISK-25067
Reported by: Matt Jordan

Change-Id: Iaa6b8d2d6bab7f99ee08e1c8908b8272a8987e65