db_test2 BackgroundPurgeTest intermittently failing on main

[Yuval-Ariel]

Describe the bug
db_test2 BackgroundPurgeTest fails:
[ RUN ] DBTest2.BackgroundPurgeTest db/db_test2.cc:5909: Failure Expected: (value) > (base_value), actual: 2048 vs 2048

To Reproduce
Steps to reproduce the behavior:
while ./db_test2 --gtest_filter=DBTest2.BackgroundPurgeTest; do sleep 1; done
get the CPU busy

System (please complete the following information):
azure default instances Standard_L16s_v2

[Yuval-Ariel]

this happens since the iterator is deleted before the sleeping task is scheduled in the LOW pri pool which causes the purge to be executed (instead of being delayed by the sleeping bg task)