Test `async` implementation of `write` hypercall

[n0toose]

See: #1145 (comment)

My work on hermit-os/kernel#2013 showed me that although we do test the write hypercall (both on Uhyve, and implicitly in the kernel as well), we don't do that for the #[async_trait] variant. This could've led to a regression, so it might make sense to add a test that uses the separate implementation that otherwise looks identical.