Fix failing AppVeyor CI

[cosmo0920]

Check CONTRIBUTING guideline first and here is the list to help us investigate the problem.

Is your feature request related to a problem? Please describe.

Currently, AppVeyor CI is always failing.
We should pay attention to AppVeyor CI.

Describe the solution you'd like

Fix AppVeyor CI with several PRs.

Describe alternatives you've considered

We should mark omittable testcases for hard to fix.

Additional context

This makes obscure whether Fluentd on Windows works well or not.

[cosmo0920]

https://ci.appveyor.com/project/fluent/fluentd/builds/31359133/job/9j76r04jtjfhclj5#L152 @i.retry is sometimes failing.
We should investigate it.

[ganmacs]

It sometimes happens in not windows env https://travis-ci.org/fluent/fluentd/jobs/655185903#L626

[cosmo0920]

Hmm..., @i.retry assertion seems to be fragile....

[cosmo0920]

Fragile testcases should cause failure in other platforms.
So, we can close this issue ticket due to fixing Windows related failures.