Fixed AppVeyor errors.

[sttk]

Since some test codes caused errors on AppVeyor, I've fixed them.

Modification:

• appveyor.yml
  Use for statement because the wildcard in npm test was not expanded. (But coverage is unavailable).
• test/completion.js
  Change stdout handling to reading a redirected file because the argument for stdout was empty on node v0.10 for Windows.
• test/flags-help.js
  Change stdout handling to reading a redirected file because the argument for stdout contained strange strings in the tail on node v0.10 for Windows.
• test/flags-tasks-json.js
  Add escaping backslashes in outputString to eliminate JSON.parse errors on Windows, and remove replacing backslashes to slashes in stdout.

[phated]

Thanks!