Check variables used in 'When' clause is not changed in the tasks in a block

Take a look at the playbooks to avoid below situation.
```
- set_fact:
      test: true
- block:
      - debug: var=test
      - set_fact:
            test: false
      - debug: var=test
   when: test
- debug:
       msg: "The above second debug will not run."
   when: not test
```