Double the fixture lock timeout

[EliahKagan]

Fixes #1605

As suggested in #1605 (comment), this increases the lock timeout gix-testtools uses when it runs fixture scripts.

If 6 minutes is too long, I can experiment with intermediate values.

Duration::from_mins is still marked experimental in the standard library, so I've continued to use Duration::from_secs with * 60 as before.

[Byron]

Thanks a lot, I am glad this works.

And I am so looking forward to the day when there is a system that allows to keep issues, and reviews, and all metadata, directly in the repository and referenceable from commits.