Tests fail intermittently

[courajs]

See https://travis-ci.org/jekyll/jekyll-compose/jobs/158906538
I've seen this locally as well.
When it happens, it also pollutes the main directory with the following:

• _drafts/
• _posts/
• a-test-page.html
• a-test-page.md
• an-existing-page.md

[jekyllbot]

This issue has been automatically marked as stale because it has not been commented on for at least two months.

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