fix(test): add missing await to unlink in save_database test

[KentarouTakeda]

What does it do?

Summary

• Fix race condition in test/scripts/filters/save_database.ts where unlink(dbPath) was not awaited
• The missing await caused the "do nothing if hexo is not initialized" test to intermittently fail because the database file from the previous test had not been deleted yet

Failure log before fix

4. Save database
  do nothing if hexo is not initialized:
 AssertionError: expected true to be false
  - expected - actual
 -true
 +false
 at Context. (test/scripts/filters/save_database.ts:30:21)

Test plan

• Ran the test 5 times consecutively with no failures

Pull request tasks

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

[github-actions]

How to test

git clone -b fix/flaky-save-database-test https://github.com/KentarouTakeda/hexojs-hexo.git
cd hexo
npm install
npm test