Intent of pending_cancel_is_valid

All 3 exact tests in this concept. Every test is marked by where it enters the hierarchy.

go.temporal.io/ 3 tests
- server/ 3 tests
  - chasm/ 3 tests
    
    lib/ 3 tests
    
    scheduler/ 3 tests
    
    TestExecuteTask_Validate/ 2 tests
    
    go.temporal.io/server/chasm/lib/scheduler/TestExecuteTask_Validate/eligible_start_is_valid Inherited eligible_start_is_valid
    
    go.temporal.io/server/chasm/lib/scheduler/TestExecuteTask_Validate/pending_cancel_is_valid Introduced here
    
    go.temporal.io/server/chasm/lib/scheduler/TestExecuteTask_ExceedsMaxActionsPerExecution Inherited invoker_tasks.go ×1