fix(test): delete /tmp/_multiconfig.yml file at the end of the test.

[dailyrandomphoto]

What does it do?

Delete /tmp/_multiconfig.yml file at the end of the test.
Otherwise, the test will fail when other users on the same server run the test.

How to test

git clone -b fix-multiconfig-test https://github.com/dailyrandomphoto/hexo.git
cd hexo
npm install
npm test

# login with another user
npm test

Screenshots

Pull request tasks

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

[coveralls]

Coverage remained the same at 97.269% when pulling 80ed8f0 on dailyrandomphoto:fix-multiconfig-test into d8ec214 on hexojs:master.