tests: Replace unsend with async-lock for LocalExecutor barrier tests

[cryptomilk]

The unsend crate uses the Patron license which is not a free-software license. This is an issue when packaging for Fedora. However looking at the usage, it is only used for a barrier.

So lets replace unsend::lock::Barrier with async_lock::Barrier (already a dev-dependency).

[fogti]

Can you provide a source for "the unsend crate uses a non-free license (Patron)"? Because afaik: https://docs.rs/crate/unsend/latest/source/Cargo.toml#52 it has license = "LGPL-3.0-or-later OR MPL-2.0" and contains mostly code by smol-rs admins and contributors, if not exclusively. The only reason to get rid of it in our dependency tree that I would deem appropriate is that is hasn't received any activity in the last 3 years.

[cryptomilk]

See the discussion at https://bugzilla.redhat.com/show_bug.cgi?id=2442346 for the details.

Currently it looks like unsend is just imported for the barrier and async-lock is providing more or less the same barrier.

If unseen does a new release you can reconsider, 3 years is already a long time without a release :-)

[notgull]

Good to do anyways, unsend is mostly unmaintained by me.

[fogti]

ah, I found out that the patron license was only removed in notgull/unsend@c407efd, but there simply was no release since then.

I think that after the actual current license of unsend is clarified, it would make sense to make a release of that purely to clear up the license situation.