test: eliminate arbitrary sleep delays in artifact DAO and gtask pool tests (#23778)

Remove 2 seconds of time.Sleep delays in dao_test.go by using explicit timestamp offsets (now.Add) for artifact creation. In pool_test.go, replace 500ms of hardcoded sleep delays with assert.Eventually polling. This accelerates test execution by 2.5 seconds and prevents test flakiness.

Signed-off-by: Norway-02 <anshulkhetade02@gmail.com>