Intent of TestSentinelIdleTask_Validate_Valid, TestIdleTask_Validate_SentinelNotHeldOpen

All 5 exact tests in this concept. Every test is marked by where it enters the hierarchy.

go.temporal.io/ 5 tests
- server/ 5 tests
  - chasm/ 5 tests
    
    lib/ 5 tests
    
    scheduler/ 5 tests
    
    go.temporal.io/server/chasm/lib/scheduler/TestIdleTask_Validate_SentinelNotHeldOpen Introduced here
    
    go.temporal.io/server/chasm/lib/scheduler/TestSentinelIdleTask_Execute Inherited TestSentinelIdleTask_Execute
    
    go.temporal.io/server/chasm/lib/scheduler/TestSentinelIdleTask_Validate_ExpirationShiftedLater Inherited TestSentinelIdleTask_Validate_ExpirationShiftedLater
    
    go.temporal.io/server/chasm/lib/scheduler/TestSentinelIdleTask_Validate_InvalidAfterClosed Inherited TestSentinelIdleTask_Validate_InvalidAfterClosed
    
    go.temporal.io/server/chasm/lib/scheduler/TestSentinelIdleTask_Validate_Valid Introduced here