Add unix domain socket based async channel test

[danieleggert]

Add unix domain socket based async channel test.

Motivation:

This adds additional test coverage, and also provides an example for how to setup async-channel based UNIX domain socket channel pairs (server + client).

Modifications:

Adds a new file with test code.

Result:

Changes to test only.

[Lukasa]

Thanks for this Daniel! In general we're happy to take a test like this, but it will need to be an XCTest, not a swift-testing test, as we still support 5.10.