Fix spurious error for test json_artifact_includes_test_flag
Fix spurious error for test json_artifact_includes_test_flag

There is a race condition for which job finishes first. On CI I am able
to reproduce the error about 1% of the time.

Recent failures:
- https://ci.appveyor.com/project/rust-lang-libs/cargo/builds/20151768/job/oghwemec7b19turs
- https://travis-ci.org/rust-lang/cargo/jobs/464717439

tshepang/cargo,tshepang/cargo,rust-lang/cargo,tsurai/cargo,vojtechkral/cargo,nipunn1313/cargo,tshepang/cargo,tshepang/cargo,tsurai/cargo,eminence/cargo,rust-lang/cargo,eminence/cargo,rust-lang/cargo,rustyhorde/cargo,rustyhorde/cargo,eminence/cargo,nipunn1313/cargo,vojtechkral/cargo,rust-lang/cargo,nipunn1313/cargo,rustyhorde/cargo,tsurai/cargo,vojtechkral/cargo,rustyhorde/cargo,nipunn1313/cargo,tsurai/cargo,eminence/cargo,vojtechkral/cargo,tsurai/cargo