chore(filter.spec): fix variable-- test case

[S-YOU]

Please check if the PR fulfills these requirements

• Do not submit PRs against the master branch.
• Checkout a topic branch from the relevant branch, e.g. dev, and merge back against that branch.
• The tests are running: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup

If adding new feature:

• Added accompanying test case.
• In the commit message, provide convincing reason to add this feature.
  Ideally you should open a suggestion issue first and have it greenlighted before working on it.

If fixing a bug:

• If you are resolving a special issue, add (fix #xxxx[,#xxx]) (#xxxx is the issue id) in your PR title
• Provide detailed description of the bug in the PR. Live demo preferred.
• Add appropriate test coverage if applicable.

What kind of change does this PR introduce? (check at least one with "x")

[ ] Bugfix [ ] Feature [ ] Code style update [x] Refactoring [ ] Build related changes [ ] Other... Please describe: 

Does this PR introduce a breaking change? (check one with "x")

[ ] Yes [x] No 

If this PR contains a breaking change, please describe the impact and migration path for existing applications:

Other information: