fix(disableNunjucks): query both async and sync versions of renderer

[curbengh]

What does it do?

A workaround to fix #3860, disableNunjucks may not be reliable in synchronous mode. The issue could be due to Promise.method() not including disableNunjucks property.

Before: https://github.com/hexojs/hexo-renderer-coffeescript/pull/72/checks?check_run_id=1030164260
After: https://github.com/hexojs/hexo-renderer-coffeescript/pull/72/checks?check_run_id=1030174895

How to test

git clone -b BRANCH https://github.com/USER/hexo.git
cd hexo
npm install
npm test

Screenshots

Pull request tasks

• Add test cases for the changes.
• Passed the CI test.

[coveralls]

Coverage increased (+0.003%) to 98.299% when pulling bdd4156 on curbengh:disable-nunjucks-sync into 08c8b23 on hexojs:master.