Fix default failures_manifest_file

[chubarovNick]

Recently, we tried to override the failures_manifest_file configuration in our CI pipeline to achieve the partitioning of our test suite on top of the same file system.

Thanks :)

[josevalim]

Thank you. Given this option does not work at all, I wonder if we should use this opportunity to rename it to _path, as that would be the more consistent name to use in Elixir. But don't worry about it, I'd rename it after merge.

[josevalim]

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